Here's a quick and dirty solution:

#!/usr/bin/perl

use strict;
use warnings;

my @lines   = ();
my %numbers = ();

while(<DATA>) {
    my ($number) = m/^(\d+)/;

    push @lines, [$number, $_];
    $numbers{$number}++;
}

foreach (@lines) {
    print $_->[1] if $numbers{$_->[0]} > 1;
}

__DATA__
1  ahjewgfje
1  gopjregre
2  kkkkkkk
3  figjiorger
3  rekopfroeer
3  ejfjviknced
4  erjgirjgerio
5  eieuiee
5  reopjtfrpeoi
[download]

What this does is iterate through the data (from the special DATA filehandle), extract the number at the beginning of each line using a regular expression (in list context, so it returns the captured values), and populate an array of arrays where each element of the first array is an anonymous two-element array containing the extracted number and the entire line. It also keeps a running total of how often each number was seen.

Once all that's done it goes through the array of arrays; for each element ($_, representing a line), it checks whether the extracted number ($_->[0], the first element of the anonymous array currently being looked at) has a running total of more than one, and if so, prints the line in question ($_->[1], the second element of the anonymous array).

One downside is that this'll slurp the entire file into memory before printing anything, which may be a problem if your files are very large.

It'll also work no matter whether lines with the same number are separated by lines with different numbers or whether they're not (as in your sample data). Whether this is a feature or a bug only you can say.