---
name: "Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic"
section: "3pm"
platform: "linux"
package: "perl-perl-critic"
version: "1.156-3"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic"
---

# Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic(3pm)

Don't pass $\_ to built-in functions that assume it, or to most filetest operators.

## NAME

Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic - Don't pass $\_ to built-in functions that assume it, or to most filetest operators.

## AFFILIATION

This Policy is part of the Perl::Critic distribution.

## DESCRIPTION

There are a number of places where `$_`, or "the topic" variable, is unnecessary.

### Topic unnecessary for certain Perl built-in functions

Many Perl built-in functions will operate on `$_` if no argument is passed.  For example, the `"length"` function will operate on `$_` by default.  This snippet:

```
    for ( @list ) {
        if ( length( $_ ) == 4 ) { ...
```

is more idiomatically written as:

```
    for ( @list ) {
        if ( length == 4 ) { ...
```

In the case of the `"split"` function, the second argument is the one that defaults to `$_`.  This snippet:

```
    for ( @list ) {
        my @args = split /\t/, $_;
```

is better written as:

```
    for ( @list ) {
        my @args = split /\t/;
```

There is one built-in that this policy does **not** check for: `"reverse"` called with `$_`.

The `"reverse"` function only operates on `$_` if called in scalar context.  Therefore:

```
    for ( @list ) {
        my $backwards = reverse $_;
```

is better written as:

```
    for ( @list ) {
        my $backwards = reverse;
```

However, the distinction for scalar vs. list context on `"reverse"` is not yet working.  See KNOWN BUGS below.

### Topic unnecessary for most filetest operators

Another place that `$_` is unnecessary is with a filetest operator.

```
    # These are identical.
    my $size = -s $_;
    my $size = -s;
    # These are identical.
    if ( -r $_ ) { ...
    if ( -r ) { ...
```

The exception is after the `"-t"` filetest operator, which instead of defaulting to `$_` defaults to `"STDIN"`.

```
    # These are NOT identical.
    if ( -t $_ ) { ...
    if ( -t ) { ...  # Checks STDIN, not $_
```

## KNOWN BUGS

This policy flags a false positive on `"reverse"` called in list context, since `"reverse"` in list context does *not* assume `$_`.

```
    my $s = reverse( $_ ); # $_ is useless.
    my @a = reverse( $_ ); # $_ is not useless here.
```

## CONFIGURATION

This Policy is not configurable except for the standard options.

## AUTHOR

Andy Lester \<andy@petdance.com\>

## COPYRIGHT

Copyright (c) 2013-2022 Andy Lester \<andy@petdance.com\>

This library is free software; you can redistribute it and/or modify it under the terms of the Artistic License 2.0.