use strict;
use warnings;

my @inserts = (
    [1, 1, 1],
    [2, 2, 2]
    );

my $outfile = 'delme.txt';
my $ins = 0;

open OUTFILE, '>', $outfile;
print OUTFILE <<DATA;
first second third
1 2 3
x y z
DATA
close OUTFILE;

insert (@$_) for @inserts;

@ARGV = $outfile;
$/ = undef;
print <>;

# OP's sub sans cat calls follows