knoppix@Microknoppix:~$ perl -E '
> $rx  = qr{(\d+)};
> 
> $str = q{ab12ef};
> $_   = q{gh34kl};
> 
> say $1 if $str =~ $rx;
> say $1 if $rx;
> say $1 if m{$rx};
> 
> $_   = q{mn56qr};
> 
> say $1 if $_ =~ $rx;'
12
12
34
56
knoppix@Microknoppix:~$