Then you need to swap the part that is searched with the part that is replaced.

Currently, as haukex already explained, you have:

#           delimiters
#             |  |  |
#             v  v  v
$localdir =~ s/\//\\/g;
#              ^^ ^^
#              |   |
#    search part   replacement
[download]

So, if you swap search part and replacement, you get:

#           delimiters
#             |  |  |
#             v  v  v
$localdir =~ s/\\/\//g;
#              ^^ ^^
#              |   |
#    search part   replacement
[download]

The above gets much clearer if you use another delimiter instead of s///g, for example, s{}{}g. Then your original line is

#           delimiters
#             |  ||  |
#             v  vv  v
$localdir =~ s{/}{\\}g;
#              ^  ^^
#              |   |
#    search part   replacement
[download]

With the swap of search part and replacement part, that becomes:

<c>
#           delimiters
#             |  ||  |
#             v  vv  v
$localdir =~ s{\\}{/}g;
#              ^^  ^
#              |   |
#    search part   replacement
[download]