my $i= 0;
$str =~ s/(...)/ 3 == ++$i ? 'fish' : $1 /ge;

[download]
or 
my $i = 3;
0 while $str =~ /fsih/gi && --$i;
substr( $str, $-[0], $+[0]-$-[0], 'fish' )   if  ! $i;

[download]

- tye        

In reply to Re: Replacing a specfied instance of a pattern in a string (2 more) by tye
in thread Replacing a specfied instance of a pattern in a string by krisahoch

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