A hint of what is going on:

use warnings;
use strict;
use Data::Dumper;

gf();
gf();

sub gf
{
    my $ins1 = '4 A -4 C -4 B 1 D';
    my @inserts;
    print "\nat the beginning: " . \@inserts . "\n";
    print $ins1, "\n";
    $ins1 =~ /[\d+-]+(?: \w ([\d+-]+)(?{print \@inserts, "\n"; push @i
+nserts, $1}))*/g;
    print "\n" . join ' ', @inserts;
}
[download]

This prints:

at the beginning: ARRAY(0x189126c)
4 A -4 C -4 B 1 D
ARRAY(0x189126c)
ARRAY(0x189126c)
ARRAY(0x189126c)

-4 -4 1
at the beginning: ARRAY(0x224fc8)
4 A -4 C -4 B 1 D
ARRAY(0x189126c)
ARRAY(0x189126c)
ARRAY(0x189126c)
[download]

The push for the second pass pushed everything into the first array.