$s = "foo img123 bar img456 baz img789 boo";
my $c = 0;
my $max = 2;
$s =~ s/(img)(\d+)/++$c > $max ? "$1$2" : qq{<img src="$2">}/eg;
print $s;
__END__
Ausgabe:
foo <img src="123"> bar <img src="456"> baz img789 boo