use Tie::RefHash;

tie my %m, "Tie::RefHash";

%m = ( qr/abc/ => "ABC", 
       qr/def/ => "XYZ", );

$str = "def";

for my $r ( keys %m ) { if( $str =~ $r ) { $str = $m{$r}; last; } }

print $str, "\n"