use strict;
my $s=n_sub ("Toy boats are for the little boys" ,2,"b","g");
$s=n_sub ($s,4,"o","i");
$s=n_sub ($s,2,"y","rl");
print $s;

sub n_sub {
    my ($os, $xth, $ic, $oc) = @_;
    my @el= split /$ic/,$os, $xth;
    $el[-1] =~ s/$ic/$oc/g;
    return join ($ic, @el);
}