Return reference to an array in Perl

There is an error in the premise of your question, because this line:

$monitored_paths = {'svn/test-repo' => '/src/cpp', '/src/test', '/src/test2'}

is the equivalent of either of these:

$monitored_paths = {'svn/test-repo' => '/src/cpp', '/src/test' => '/src/test2'}
$monitored_paths = {'svn/test-repo', '/src/cpp', '/src/test', '/src/test2'}

What you really want is:

$monitored_paths = {'svn/test-repo' => ['/src/cpp', '/src/test', '/src/test2']}

where [] denotes an array reference. You create an array reference like this:

my $arrayref = [1, 2, 3]; # called an "anonymous array reference"

or like this:

my @array = (1, 2, 3);
my $arrayref = \@array; 

You want something such as:

$monitored_paths = { '/svn/test-repo'  => 'http://....list.txt' }
foreach my $key (keys %$monitored_paths) {
    next if ref $monitored_paths{$key} eq 'ARRAY'; # skip if done already
    my @paths = get_paths_from_url($key);
    $monitored_paths->{$key} = \@paths; # replace URL with arrayref of paths
}

replacing get_paths_from_url with your URL-fetching and parsing function (using LWP or whatever...since that was not really part of your question I assume you already know how to do that). If you write your function get_paths_from_url to return an array reference in the first place instead of an array, you can save a step and write $monitored_paths->{$key} = get_paths_from_url($key) instead.

If you want to read from the file and add each path to the array you could do something like:

use strictures 1;

my $monitored_paths = {};
open( my $FILE, '<', '/path/to/file' ) or die 'Unable to open file '. $!;
while($FILE){
    push @{ $monitored_paths->{'svn/test-repo'} }, $_;
}

use LWP::Simple;
my $content = get($url); ## do some error checking
$monitored_paths = {'svn/test-repo' => [split( "\n", $content)]}

use LWP;
foreach (keys %monitored_paths)
{
   my  $content = get($monitored_paths{$_});# Perform error checking
   $monitored_paths_final {$_} = join(",",split("\n",$content));
}