use warnings;
use strict;
my $a = 'L:\\Data\\5555\\';


$a =~ s#l:\\Data\\([0-9]*)\\#D:\\Sites\\$1\\Data\\#i;

print $a;
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Unrecognized escape \D passed through at FILENAME line NUMBER.
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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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-------------------------------------
Nothing is too wonderful to be true
-- Michael Faraday

If @file is an array then you want a for loop, not a while loop. If you want to read lines from a file into $_ then you want while ( <FILE> ) {

I.e., you do have an infinite loop, @file has a constant number of things in it, so it is always true, unless you shift or pop things from it in the loop. (but a for loop is probably more appropriate).

([0-9]*) will match zero or more digits.

To be a little cleaner, you probably should match 1 or more digits using ([0-9]+)