$ perl -E 'my $x = "AAA"; $x =~ s/A/B/; say $x'
BAA

$ perl -E 'my $x = "AAA"; $x =~ s/A/B/ for 0 .. length($x) - 1; say $x'
BBB

$ perl -E 'my $x = "AAA"; $x =~ s/A/B/g; say $x'
BBB