I'm really actually quite interested in why you need to do this. Generally speaking, when you load a string from a file, the perl interpreter doesn't get involved, so you end up with a literal @ in the string, and there's no need to escape it.

That's not to say that this isn't a valid requirement, only that, without information to the contrary, the likelihood of a real desire to escape this character is really quite low. Not zero, but still quite low.

If you are sending mail with a script, and have something like print(MAIL "To: someone\@somewhere.someorg\n"); the @ needs to be escaped. Of course, one could use single quotes here (except for the newline), but one might want to store the address in a variable and have that and some other things interpolated, and then the @ may cause problems if it's not escaped. It is often possible to get around that with some cleverness, but sometimes it's easier to just escape the troublesome characters.
(At least, that's what I imagine the OP might be dealing with.)
(Update: After thinking about it for a while, I can't really think of a convincing reason to escape the @. My brain is just not working well today...please ignore me! I'll shut up for the rest of the day.)

$$email[0] =~ s!@!\\@!;
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my $mailaddress='ganesans@uic.com';

$mailaddress=~s/\@/\\\@/;
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#!/usr/bin/perl -w

use strict;

my $email = 'user@company.com';

# this will replace all @'s - add option of (2) to split to replace on
+ly one

print join( '\@', split( '\@', $email ) ), "\n";

# this will replace only one - add g to replace all
$email =~ s/@/\\@/;

print $email, "\n";
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