Don't know if this is true just of Win32 (running ActivePerl 5.6.1 (633)) but if you use quotes in the shebang line it doesn't work.

i.e.

#!/usr/bin/perl -I"/somedir/"
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..won't work but

#!/usr/bin/perl -I/somedir/
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.. will.

The /usr/bin/perl wouldn't be strictly correct on Win32, but under ActivePerl it doesn't seem to matter (could set it to the proper DOS path if required).