in reply to Using previous match in the same matching regexp

You could do: 
$str =~ /^(\d+) /g and $str =~ /\G(.{$1})/;

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Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In Section Seekers of Perl Wisdom