The problem is that your test is wrong. Try this:

$ perl -le '$out = "+44"; $in=quotemeta $out; $out =~ s/$in//; print "
+Result: |$in|$out|\n";'
Result: |\+44||
[download]

Note that '$in' is the pattern, not the original value of '$out'.

Alternatively, you may write:

$ perl -le '$in = "+44"; $out = $in; $out =~ s/\Q$in//; print "Result:
+ |$in|$out|\n";'
Result: |+44||
[download]