Amazingly, one of the solutions worked perfectly.

my $regexp = "..g.{18}c";
my @antisense_oligos;
$_ = $input_seq;
do{
    push @antisense_oligos, $1 if (m/^($regexp)/)
} while ( s/^.// );
$,="\n";
print @antisense_oligos;
[download]

When I get the chance, I'll try to put pos() to work.

Thank you!!!