Use a hash:

my %freq = (
    a => 123,
    b => 200,
    c => 700,
    ...
    z => 800,
);

my $sum = 0;
for (split //, $word) {
    $sum += $freq{$_};
}
print "Sum: $sum\n";
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Note that the lookup is case sensitive, so may you need lc or uc to fix that.

See perlintro and perldata.