A bit of debugging would tell you that your filename is probably not what you think it is and that your transliteration is not doing what you think:

my $filename = "C:\\Invoices\\foobar.pdf";
print "1 : >>$filename<<\n";
$filename =~ tr!\\!/!s;
print "2 : >>$filename<<\n";
__END__
1 : >>C:\Invoices\foobar.pdf<<
2 : >>C:/Invoices/foobar.pdf<<
[download]

It might also be useful to include file test in your code, just to check that it isn't a permissions problem too.

HTH.