bad-while.pl

#! /usr/bin/env perl

use strict;
use warnings;

my $s = "foo\nbar\n";
open my $fh, '<', \$s;

# this while is OK, because it's top-level
while (<$fh>) { }

sub fn_good {

    # this while is OK too, because it localizes
    local $_ = undef;
    while (<$fh>) { }
    return;
}

sub fn_good_too {

    # this while is OK too, because it binds
    while ( my $x = <$fh> ) { }
    return;
}

sub fn_bad {

    # this while is bad, because it modifies the outer topic
    while (<$fh>) { }
    return;
}

my @data = ( 42, 43 );
fn_bad() for @data;
die "My data is gone!\n" unless $data[0];