sub prog_replace_string {
  my ($replace_this, $with_this, $string) = @_;
  $string =~ s{\Q$replace_this\E}{$with_this}g;
  $string;
}

##</code><code>##

sub replace_vars {
  my ($string, $hash) = @_;
  $string =~ s{\$(\w+)}{$$hash{$1}}ge;
  $string;
}

my %vars = ( a => 1, b => 22, c => 333 );
print replace_vars('a: $a, b: $b, c: $c', \%vars);