delete removes the entire entry, i.e. key plus value, while undef just sets the value to undef, leaving the key in place:

#!/usr/bin/perl

use Data::Dumper;

my %hash = ( key => 'foo', key2 => 'bar');

print Dumper \%hash;

undef($hash{key});

print Dumper \%hash;

delete($hash{key});

print Dumper \%hash;

__END__
$VAR1 = {
          'key2' => 'bar',
          'key' => 'foo'
        };
$VAR1 = {
          'key2' => 'bar',
          'key' => undef
        };
$VAR1 = {
          'key2' => 'bar'
        };
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