Maybe you should close $P4DIFF before you open the associated file?

Depending on how much you've written, the IO buffer (size 4k) might not have been flushed yet, so the file behind $DATA could be empty.  In other words, you'd get EOF with the loop quitting right away...

This doesn't print anything (i.e. while loop not entered):

open(my $P4DIFF, '+>', "p4diff.log") or die $!;
print $P4DIFF "foo\n" x 3;
# close $P4DIFF;

my $input = "p4diff.log";
open my $DATA, '<', $input or die "could not open '$input' $!"; 
      
while (my $line = <$DATA>) {
    print ">> $line";
}
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while if you uncomment the close, you'd get:

>> foo
>> foo
>> foo
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