We are good at giving advice. ;)

First, always use strictures (quite likely you are in fact, but didn't include them in your sample).

Use the three parameter version of open and use lexical file handles:

open my $FILE1_TEMP, '+>', $file1_temp or die "Could not open $file1_t
+emp: $!";
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However, that is probably not the advice you want. Your code appends a string to a file then prints everything from the end of the file to, well, the end of the file - that is, nothing (as you have noticed).

The fix is either to close and reopen the file, or to seek to the start before your while loop:

use strict;
use warnings;

my $file1_temp = "delme.txt";

open my $ioFile, '+>', $file1_temp or die "Could not open $file1_temp:
+ $!";
print $ioFile "hello, world!";

seek $ioFile, 0, 0;
print while <$ioFile>;
close $ioFile;
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