#!/usr/bin/perl

use strict;
use warnings;

my @List = (1,2,3,4,5,6,7,8,9,8,7,6,5,4,3,2,1);
my $Tag = 5;

my @Result = ();

while (@List) { push @Result, splice @List, 0, 3 if shift @List == $Tag }

use Data::Dumper;
print Data::Dumper->new([\@Result], [qw[Result]])->Indent(0)->Dump,"\n";

__END__