#assuming char width values in inches
my %char_width(a=>1, 
                b=>1.5); #etc.etc.

#pseudo-code
my $cell_width;

while(length($string) != 0)
{
   my $char = ($string =~ s/^(.{1})//);
   
   $cell_width += $char_width{$char};
};

$cell_width /= 72;
   

##</code><code>##

my $cell_width = length($string) * $char_width;