Here's a better version. I was going to wipe my scratchpad and saw that I could do one thing better ... then another ... then realized that there didn't need to be an identified element per subgraph...

#!/usr/bin/env perl

use strict;
use warnings;
use List::MoreUtils(qw(uniq));

my @array = map {[split]} ("11 12", "11 13", "9 8 7", "3 4", "11 4");

my %g = ();

for my $i (@array) {
    my @v = map {@$_} uniq map {$g{$_} or [$_]} @$i;
    @g{@v} = (\@v) x @v;
}

print join ("\n", map {join " ", sort {$a<=>$b} @$_} uniq values %g), 
+"\n";
[download]

Same output as above, although now the ordering of the lines is fortuitous. (But obviously can be set, if desired, by another sort.)

Hopefully it's a bit more clear that, since each iteration creates a complete set of associated vertices, the process finds the sets of vertices of the connected subgraphs of an arbitrary graph.

(Please pardon the enthusiasm, this is the kind of stuff I like to think about.)