valanp,
first of all, you have changed the original content of your node without leaving notice. This will confuse future readers. Please don't do that.

As for your problem: next time please include complete sample data and input/expected output specs. In your OP there was only one special case.

Anyway, here is a subroutine for you that solves your problem:

use warnings;
use strict;

use Data::Dumper;

my %value = (
    A => 2.74,
    B => 2.64,
    C => 2.5,
);

my %factor = (
    A => 50/100,
    B => 55/100,
    C => 60/100,
);

sub calc
{
    # fetch arguments
    my $expression = shift;
    my $values     = shift;
    my $factors    = shift;

    # build an array of normalized terms
    # (normalize = replace non existant factors by 1)
    # as in 4ABC => 4A 1B 1C
    my @term;

    while ( $expression =~ /([0-9]*)([A-Z])/g )
    {
        push @term, [$1?$1:1, $2];
    }

    # now iterate over that array and do the 
    # calculation
    my $result = 1;

    $result *= $_->[0] * $values->{$_->[1]} * $factors->{$_->[1]}
        for @term;

    return $result;
}
    
print calc ("4ABC",   \%value, \%factor), "\n";
print calc ("2A2B2C", \%value, \%factor), "\n";
print calc ("3C",     \%value, \%factor), "\n";
print calc ("ABC",    \%value, \%factor), "\n";
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