I didn't give enough context. My bad. The substitution does seem to work. Here's more context, and the error.

  while ( @file ) {
    s#l:\\Data\\([0-9]*)\\#D:\\Sites\\$1\\Data\\#i;
    print;
  }
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@file is filled with many lines like so: my @file = <FILE>;

Here are the warnings:

Use of uninitialized value in print at foo.pl line 44.
Use of uninitialized value in substitution (s///) at foo.pl line 43.
Use of uninitialized value in substitution (s///) at foo.pl line 43.
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