One way: 
my $foo = 'Fsih my test variable is fsihd or is it gfsih or gfsihd fsi
+h';
# Replace the 3rd
$foo =~ s/((?:fsih.*?){2})fsih/$1fish/i or warn "Not found";
print "Foo = $foo\n";

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Caution: Contents may have been coded under pressure.

In reply to Re: Replacing a specfied instance of a pattern in a string by Roy Johnson
in thread Replacing a specfied instance of a pattern in a string by krisahoch

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