If you need a solution that doesn't clobber $string: 
$substr = $string =~ /^(\d+) (.*)/ && substr($2, 0, $1);

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If you need both $len and $substring: 
($len, $substr) = $string =~ /^(\d+) (.*)/ && ($1, substr($2, 0, $1));

[download]

In reply to Re: Using previous match in the same matching regexp by ikegami
in thread Using previous match in the same matching regexp by cyberconte

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