If you're starting with a multi-line string (1^a^x\r\n2^b^y\r\n3^c^z\r\n...) stored in a scalar variable ($itDetail), then the easier way would be:

open( FH, ">/some/path/debug.txt" ) or die "can't write debug.txt: $!"
+;

for my $_line ( split /[\r\n]+/, $itDetail ) {
    my ( $Num, $Qt, $_trashit ) = split /\^/, $_line;
    print FH "Num = $Num\n";
    # do other things...
}
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