use strict; use warnings;

my $file = "test.meg";
my $mb   = 1024 * 1024;
my $meg  = '0' x $mb;

open my $fh, '>', $file or die "yuck, $file: $!\n";
print $fh $meg;
seek  $fh, 0, 0;  # rewind to print at start. I guess you want this
print $fh "I Write\n";
close $fh;