sort returns the sorted list. It doesn't modify the argument.

@KEYS = sort(@KEYS);
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Furthermore, since you're converting the timestamps to numbers, you want to do a numerical comparison instead of a lexical comparison.

@KEYS = sort { $a <=> $b } @KEYS;
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But you could simply use $Elements[5] as the keys and a lexical comparison.

Update: By the way, here's another way of doing what you want (fast):

my @triee = map { substr($_, 8) }
            sort
            map { (split(/;/, $_))[5] . $_ }
            <INFILE>;

foreach my $triee (@triee) {
   print OUTFILE $triee;
}
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