I would appreciate any input on 1) code correctness

use strict;
use warnings;

open(IN, "list.txt") or die;
chomp($line = <IN>);
($a, @ss) = split /\s+/, $line;
while (chomp($line = <IN>)) {
    ($aa, @prs) = split /\s+/, $line;
    for ($i=0; $i<@prs; $i++) {
$s{$aa}{$ss[$i]} = $prs[$i];
    }
}
close(IN);
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I would write that code as:

use strict;
use warnings;

open my $IN, '<', 'list.txt' or die "Cannot open 'list.txt' because: $
+!";
my ( undef, @ss ) = split ' ', <$IN>;
my %s;
while ( <$IN> ) {
    my ( $aa, @prs ) = split;
    @{ $s{ $aa } }{ @ss } = @prs;
}
close $IN;
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