This is what I used, and yes it does work.

#!/usr/bin/perl
$a = "abc";
$b = "def";
$c = "abc leppard";

$c =~ s/$a/$b/;
print $c;
[download]

I didn't think you could transliterate on that so I tried it and then tried a substitution. To my avail, it worked.
Just so all these other monks that think a transliteration will work, this is what I got when I ran it as  tr/$a/$b/;

bbc leppbrd
[download]

Which, I doubt is what he wanted.

PS: May I ask what ~= is? I think you mean =~. :)

UPDATE: It seems that everyone likes to answer this in a minute's time. Chady and some other monk already beat me to it :)

Tiptoeing up to a Perl hacker.
Dave AKA damian