#!/usr/local/bin/perl -w
use strict;
my $s = "Terence and Philip are sweet\n";
my $c = 'e';
my $r = 1;
my $n = 3;

sub mine
{
 my ($str,$chr,$new,$nbr)=@_;
 my $reg="(.+?)$chr" x --$nbr;
 $_=$str;
 (/$reg/)&&($str=$&)&&($_=$')&&(s/$chr/$new/g);
 return $str.$_;
}
print mine($s,$c,$r,$n);
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#!/usr/local/bin/perl -w
use strict;

my $s = "Terence and Philip are sweet\n";
my $c = 'e';
my $r = 1;
my $n = 3;

sub mine
{
 my ($str,$chr,$new,$nbr)=@_;
 my $reg="(.+?)$chr" x $nbr;
 $_=$str;
 (/$reg/)&&($str=$&)&&($_=$')&&(s/$chr/$new/g);
 return $str.$_;
}

sub bbfus {
  my ($str, $chr, $rep, $nth) = @_;
  my $pos = 0;
  $pos = index($str, $chr, $pos+1) for(1..$nth);
  substr($str, $pos) =~ s/$chr/$rep/g;
  return $str;
}

sub quents {
  my ($str, $chr, $rep, $nth) = @_;
  my $i = 1;
  $str =~ s/($chr)/$i++<$nth?$1:$rep/eg;
  return $str;
}

use Benchmark qw( timethese cmpthese );

cmpthese(500000, {
  "BBFU'S"    => sub { bbfus ($s, $c, $r, $n) },
  "Quent's" => sub { quents ($s, $c, $r, $n) },
  "MINE" => sub { mine ($s,$c,$r,$n) },
});
[download]

Benchmark: timing 500000 iterations of BBFU'S, MINE, Quent's...
    BBFU'S: 20 wallclock secs (19.45 usr +  0.00 sys = 19.45 CPU) @ 25
+706.94/s (n=500000)
      MINE: 19 wallclock secs (19.87 usr +  0.02 sys = 19.88 CPU) @ 25
+146.69/s (n=500000)
   Quent's: 25 wallclock secs (24.97 usr +  0.00 sys = 24.97 CPU) @ 20
+026.70/s (n=500000)
           Rate Quent's    MINE  BBFU'S
Quent's 20027/s      --    -20%    -22%
MINE    25147/s     26%      --     -2%
BBFU'S  25707/s     28%      2%      --
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