#!perl -w

use strict;

my $template = ' ' x 60;

sub insert {
  my (undef, $field, $position) = @_;
  substr($_[0], $position, length($field)) = $field;
}

my $record = $template;
insert($record, 'abc', 0);
insert($record, 'ghi', 6);

print "[$record]\n";