use strict;
use warnings;

my @vor = qw(a b c);
my @nach = qw(x y z);
my @Liste = qw(7 6 5 4 3);

my $re = qr(^6$);

my @Neu = map {
    /$re/
    ?
    (@vor, $_, @nach)
    :        
    ($_)
} @Liste;

print @Neu, $/;