Here's an example of what I was suggesting.

my $current_name = '';
my %extensions   = ();

while ( my $line = <DATA> ) {
  chomp $line;
  my( $name, $ext ) = split ' ', $line;
  if( $name ne $current_name ) {
    print "($current_name) => [", join( ', ', sort keys %extensions ),
+ "]\n"
      if length $current_name;
    $current_name = $name;
    %extensions   = ();
  }
  $extensions{$ext}++;
}

print "($current_name) => [", join( ', ', sort keys %extensions ), "]\
+n";

__DATA__
/foo/bar/baz/123 aaa
/foo/bar/baz/123 aab
/foo/bar/baz/123 aac
/foo/bar/baz/124 aaa
/foo/bar/baz/124 aab
[download]

The memory footprint will only be as big as the maximum number of extensions per name.