Perl default variable

$_ is the default variable, or the “topic”.

• in while loop reading from file handle, $_ will have current line
• chomp, =~ (even without the operator) work on $_
• say and print work on $_
• if no second parameter given, split works on $_
• if foreach doesn’t get iterator variable name, it uses $_
• if ($line = /regex/) is the same as if ($line = $_ =~ /regex/)

Once you have to explicitly write out $_, just use a meaningfully named variable. For example:

# Don't do this
while (<$fh>) {
    my $line = $_;
    ...
}

# Do this instead
while (my $line = <$fh>) {
    ...
}