You're missing an \\ escape at $1\Data. Try: 
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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recent perls with warnings and strict enabled would have given you a warning: 
Unrecognized escape \D passed through at FILENAME line NUMBER.

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"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re: Escaping confusion in RE... by Joost
in thread Escaping confusion in RE... by freddo411

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