use warnings;
use strict;

my $hdr = <DATA>;
my @cols = split /\s+/, $hdr;
my %hash; # pick a better name
while (<DATA>) {
    my ($row, @vals) = split;
    for my $i (0 .. $#vals) {
        $hash{$row}{$cols[$i+1]} = $vals[$i];
    }
}
print "$hash{aaa}{baz}\n";

__DATA__
Log    foo    bar    baz
aaa    123    456    789
bbb    987    654    321