$ cat t.pl
#!/usr/bin/perl
use strict;
use warnings;
use autodie;

open my $FH, '>', 'TEST.x';
print $FH <<EOTXT;
now is the time
for all good men
to come to the aid
of their party.
EOTXT
close $FH;

open $FH, '+>>', 'TEST.x';
while (<$FH>) {
    chomp;
    print "$.: $_\n";
}
print $FH "foobar\n";
close $FH;

$ perl t.pl

$ cat TEST.x
now is the time
for all good men
to come to the aid
of their party.
foobar