The issue here is not specifically with the magical $_ var, but with aliasing. Perl has a number of cases where variables are aliased, but it's not always clear when this occurs. I consider this to be a feature, and not a bug, but some would disagree. Here are some examples:

# poorly implementing VBScript's 'trim' function:

trim( $somevar );

sub somevar {
    $_[0] =~ s/^\s*//;
    $_[0] =~ s/\s*$//;
}
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Ignoring, for the moment, that this is poor code (it probably should return the new value rather than altering the argument - don't get me started on chomp and chop :), this works because the values in @_ are aliases to the original values passed to them.

Another example:

my @array = qw/ 1 2 3 4 5 /;
my @new_array = sort { $a++ <=> $b-- } @array;
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It looks innocent enough, but @array is now set to: qw/ 0 2 3 4 6 /. Oops! That's probably not what we intended.

You also have the same behavior with $_ in map and grep operators. Even though $_ is involved here, it's a case of needing to be aware of when Perl is aliasing variables, as opposed to copying them

Cheers,
Ovid

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