On the face of it something as simple as:

use strict;
use warnings;

my %hash = (a => 1, b => 0, c => 3);

while (<DATA>){
    chomp;
    s/(\w+)/\$hash{$1}/g;
    
    my $ans = eval "$_";
    
    if ($@) {
        print "Eval of $_ failed:\n   $@\n";
    } else {
        print "$_ = $ans\n";
    }
}

__DATA__
a/b
b+c
c*a/(b+a)

[download]

which prints:

Eval of $hash{a}/$hash{b} failed:
   Illegal division by zero at (eval 10)[noname.pl:10] line 1, <DATA> 
+line 1.

$hash{b}+$hash{c} = 3
$hash{c}*$hash{a}/($hash{b}+$hash{a}) = 3

[download]

does what you want. So where is the tricky part? Why do you need subs? If you need to reuse the expression store away the "parsed" version.

DWIM is Perl's answer to Gödel

In reply to Re: Parse user-entered expressions into subs for an awk-like program by GrandFather
in thread Parse user-entered expressions into subs for an awk-like program by xaprb

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