There's no need to change directories, though you can do it with the chdir command.

Here's some code that will do what you want:

#!/usr/bin/perl
use warnings;
use strict;

#
# Open file for reading (explicit), die on error
#
open SERVICES, '<', '/etc/services'
    or die "cannot open /etc/services: $!";

#
# Read thru each line (put into $_), and print the captured
# comment text if the line has a comment (anything after a #
# to the end of line).  Note that this doesn't include the 
# hash (pound, octothorpe, et al.); move it inside the parens
# for that behavior.
#
while (<SERVICES>) { print "$1\n" if /#(.*)$/ }

# Close filehandle
close SERVICES;
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Update Note in comment about not printing the hash mark.