Dear Perl monks, the other day I stumbled upon the issue that in a particular program declaring a variable with "my" actually slowed down program execution (for comparison I just repeated the very same instruction several times): 
    while (chomp ($line = <$gsr>)) {
        my @l1;
        @l1 = split $tab, $line;
        @l1 = split $tab, $line;
        @l1 = split $tab, $line;
        print(join($tab,@l1[@okc]));
        print(join($tab,@l1[@okc]));
        print(join($tab,@l1[@okc]));
    }

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this leads to following timings: 
73µs     my@l1;
1.71s    @l1 = split $tab, $line;
2.40s    @l1 = split $tab, $line;
2.33s    @l1 = split $tab, $line;
805ms    print(join($tab,@l1[@okc]));
798ms    print(join($tab,@l1[@okc]));
1.66s    print(join($tab,@l1[@okc]));

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i.e. the 2nd and following assignments seem to take more time than the 1st one. Could that be attributed to some additional overhead? It doesn't happen when omitting the "my" declaration or using "our" instead (see below). This is consistent across runs. If introducing "my" in the middle of a block the effect is the same (see at bottom). Using "local" instead of "my" seems not as expensive. But both "my" and "local" seem to slow down last write operation (is that some artifact? All measurments taken on Windows 7 with outpout redirected to a file). 
235µs    local @l1;
1.18s    @l1 = split $tab, $line;
1.79s    @l1 = split $tab, $line;
1.78s    @l1 = split $tab, $line;
824ms    print(join($tab,@l1[@okc]));
806ms    print(join($tab,@l1[@okc]));
1.61s    print(join($tab,@l1[@okc]));

92µs     our @l1;
1.77s    @l1 = split $tab, $line;
1.77s    @l1 = split $tab, $line;
1.77s    @l1 = split $tab, $line;
829ms    print(join($tab,@l1[@okc]));
820ms    print(join($tab,@l1[@okc]));
1.03s    print(join($tab,@l1[@okc]));

         #my @l1;
1.78s    @l1 = split $tab, $line;
1.77s    @l1 = split $tab, $line;
1.77s    @l1 = split $tab, $line;
810ms    print(join($tab,@l1[@okc]));
805ms    print(join($tab,@l1[@okc]));
1.01s    print(join($tab,@l1[@okc]));

1.80s    @l1 = split $tab, $line;
1.70s    my @l1 = split $tab, $line;
2.41s    @l1 = split $tab, $line;
807ms    print(join($tab,@l1[@okc]));
794ms    print(join($tab,@l1[@okc]));
1.65s    print(join($tab,@l1[@okc]));

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In reply to "my" slowing down programs? by jf1

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