How about this snippet of code which seems to do what you need ...

my @a = ( 'a1', 'b2', 'c3', 'd4' );
my @b = ( '3', '4', '5', '6' );

my @results;
foreach my $num (@b) {
    push @results, grep { /$num/ } @a;
}

#   @results will contain 'c3' and 'd4'
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Update - Erk, modified answer to address the question at hand - If in fact you do simply need to find the union of two arrays, this node should help you out :-)