hash{X}=array[X] results in two hash keys

lucastas has asked for the wisdom of the Perl Monks concerning the following question:

I coded the following myscript.pl …

#!/usr/bin/perl
$|=1;

my %arguments;

for( my $arg = 0 ; $arg <= $#ARGV ; $arg++ ) {
    print $ARGV[$arg]."\n";
    $arguments{ $arg } = $ARGV[$arg];
    foreach my $arg ( %arguments ) {
        print "key = >$arg< value = >$arguments{$arg}<\n";
    }
}
[download]

If I run the script like so…

myscript.pl arg1 arg2 agr3

I get the following output.

arg1
key = >0< value = >arg1<
key = >arg1< value = ><
arg2
key = >0< value = >arg1<
key = >arg1< value = ><
key = >1< value = >arg2<
key = >arg2< value = ><
arg3
key = >0< value = >arg1<
key = >arg1< value = ><
key = >1< value = >arg2<
key = >arg2< value = ><
key = >2< value = >arg3<
key = >arg3< value = ><

If I include use strict, I get the error "Use of uninitialized value in concatenation (.) or string at ./myscript.pl line 13." preceeding the print of the element that has the value as the key and no value.

I went a googling and did a search here at the Monistary but was unable to find an answer to the mystery. Can you help me to understand why the hash has a key/value pair of the real key and value, then a key/value pair with the real value as the key and no value to go with it?

Comment on hash{X}=array[X] results in two hash keys Download Code

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Re: hash{X}=array[X] results in two hash keys by choroba (Cardinal) on Jul 13, 2016 at 22:29 UTC
The assignment is OK. The problem lies in the way how you iterate over the hash: `foreach my $arg ( %arguments ) {` [download] You want to only iterate over the keys: `foreach my $arg ( keys %arguments ) {` [download] Without `keys` , a hash behaves like a flattened list of key-value pairs, i.e. `key1, value1, key2, value2, ...` [download] With warnings on, you'd have been warned about the uninitialized values for some "keys" (i.e. the real values): `Use of uninitialized value in concatenation (.) or string at ...` [download] ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]
Re: hash{X}=array[X] results in two hash keys by kennethk (Abbot) on Jul 13, 2016 at 23:49 UTC
If I include use strict, I get the error "Use of uninitialized value in concatenation (.) or string at ./myscript.pl line 13." preceeding the print of the element that has the value as the key and no value. <pedantic>That comes from warnings not strict. Errors from strict are fatal, and the output you got above didn't kill the script</pedantic> #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply]