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$_ = 'abcdegedfc'; # Try it without the g, too.
my $re;
$re = qr/((.)(?:(??{$re})|.?)\2)/;
print "$1\n" while (/(?=$re)/g);

$_ = 'abcdegedfc';
print "$1\n" while (/(?=((.+).?(??{quotemeta reverse $2})))/g);