my $str = '<!-- USER 20 - donkey_pusher_6 -->';
my $data;
my $re = qr/--\s*USER\s+\d+\s*-\s*(\w+)/;
my ($start, $end);

sub by_re_noback {
  ($data) = ($str =~ / ^ (?>\s*) <!-- (?>\s+) USER (?>\s+) (?>\d+) (?>\s+) - (?>\s+) (\S+?) (?>\s+) --> (?>\s*) $ /ix);
}

sub by_re {
  ($data) = ($line =~ m/<!-- USER \d+ - (\S+)/i);
}

sub by_re_comp {
  ($data) = ($str =~ $re);
}

sub by_substr {
  $end = rindex($str, ' ');
  $start = rindex($str, ' ', $end - 1);
  $data = substr($str, $start + 1, $end - $start);
}

timethese (100000, {
		  subst => \&by_substr,
		  re_comp => \&by_re_comp,
		  re => \&by_re, 
		  re_noback => \&by_re_noback,

});
--results--
Benchmark: timing 100000 iterations of re, re_comp, re_noback, subst...
        re:  1 wallclock secs ( 0.46 usr +  0.00 sys =  0.46 CPU) @ 217391.30/s (n=100000)
   re_comp:  4 wallclock secs ( 4.35 usr +  0.00 sys =  4.35 CPU) @ 22988.51/s (n=100000)
 re_noback:  6 wallclock secs ( 6.27 usr +  0.00 sys =  6.27 CPU) @ 15948.96/s (n=100000)
     subst:  1 wallclock secs ( 1.40 usr +  0.00 sys =  1.40 CPU) @ 71428.57/s (n=100000)