Your error must lie elsewhere, as the following code works for me:

#!/usr/bin/perl
use strict;

my $string = 'The price for one night is 10$, the price for two nights
+ is 100$, cash!';
print "String before:\n";
print "$string\n";

$string =~ s!\$! dollar!g;

print "String after:\n";
print "$string\n";
[download]