I tried the following program under Perl 5.005_03, 5.6.1, and 5.8.4 and got the same results - no "memory effect":

use strict;
#use warnings;

for my $s ( qw( nob bob cob bob dob ) )
{
    my $name = $1 if $s =~ /(bob)/;
    warn $name || 'undef';
}
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result:

undef at - line 7.
bob at - line 7.
undef at - line 7.
bob at - line 7.
undef at - line 7.
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