Using rindex / substr is far better than messing around with regex. For instance:

my $string = "abababa";
my $from = 'b'; my $to = 'x';
print "was [$string]\n";
substr($string, rindex($string, $from), length($from)) = $to;
print "now [$string]\n";
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$string, $from, and $to can be pretty much any length you want.