Another way :

#!/usr/bin/perl -w
use strict;

print ' AB in 10 base => ', base10('AB'), "\n";
print ' IV in 10 base => ', base10('IV'), "\n";

sub base10{
    my ( %base26, @letters, $sum );
    @letters = reverse split '', shift;
    @base26{ 'A'..'Z' } = 1..26;
    $sum += $base26{ $letters[$_] } * 26**$_ for 0..$#letters;
    return $sum;
}
__END__ 

 Output :
 AB in 10 base => 28
 IV in 10 base => 256
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hth,
PooLpi