In addition to escaping the sigil in  $foo in double-quotish interpolation (e.g., "\$foo"), another way to get the effect you describe is to use single-quotes as the  s/// replacement delimiters:

>perl -wMstrict -le
"my $foo = 'stuff';
 my $str = 'xxx XXYY yyy';
 $str =~ s'XXYY'$foo';
 print qq{'$str'};
"
'xxx $foo yyy'
[download]

But of course single-quotes are not what you have in your OP. Are you fooling us — or yourself?