#!/usr/bin/perl

use strict;
use warnings;

my $file = '/path/file.dat';
my $exists = ( -e $file );
my $normal = ( -f $file );
$exists and (not $normal) and die "$file is unsuitable for this program";
my $iocmd = $normal ? "<$file" : ">$file";

# until now we could check the existence of the file
# and how to open it and we did not need BEGIN

open my $fh, $iocmd or die "$!, for $file";
my @content = $normal
    ? <$fh> # only now is the file read in if exists
    :( "I am\n", "the default\n", "contents\n" );
$normal or print $fh @content; # or printed to otherwise
close $fh or die "$!, for $file";

# now both the file and @content contain either tbe
# program default or previously stored content depending on 
# whether the file existed.