Well, I updated the code a little bit, getting rid of the $i == 0 while loop and other stuff. But don't take my word for it: look at the code!

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

print "I will calculate whatever\n";
print "you specify according to\n";
print "Ohm's law. What shall I\n";
print "calculate? Type either\n";
print "voltage, current, or\n";
print "resistance.\n";

my $calculation = 1;
while($calculation)
{
    $calculation = <STDIN>;
    chomp $calculation;
    
    if($calculation =~ m/voltage/)
    {
        print "Type the current of the circuit.\n";
        print "Add mA at the end of your answer\n";
        print "if your answer is in milliAmps.\n";
        
        my $current = <STDIN>;
        chomp $current;
        
        print "Now type the resistance of the circuit.\n";
        
        my $resistance = <STDIN>;
        chomp $resistance;
        
        IF: if($current && $resistance)
        {
            if($current =~ m/mA/)
            {
                $current =~ s/mA//;
                
                $current /= 1000;
            }
            else
            {
                
            }
            
        }
        else
        {
            print "Those aren't valid answers.\n";
            print "Please type that again, will ya?\n";
            
            print "Type the current of the circuit.\n";
            print "Add mA at the end of your answer\n";
            print "if your answer is in milliAmps.\n";
            
            $current = <STDIN>;
            chomp $current;
            
            print "Now type the resistance of the circuit.\n";
            
            $resistance = <STDIN>;
            chomp $resistance;
            
            goto IF;
        }
        
        my $voltage = $current * $resistance;
        
        print "Ohm's law is V = IR, so the voltage of\n";
        print "the circuit is $voltage volts.\n";
        
        $calculation = 0;
    }
    elsif($calculation =~ m/current/)
    {
        print "Type the resistance of the circuit.\n";
        
        my $resistance = <STDIN>;
        chomp $resistance;
        
        print "Now type the voltage of the circuit.\n";
        
        my $voltage = <STDIN>;
        chomp $voltage;
        
        IF: if($resistance && $voltage)
        {
            
        }
        else
        {
            print "That is not a valid answer.\n";
            print "Please type that again, will ya?\n";
            
            print "Type the voltage of the circuit.\n";
            
            $resistance = <STDIN>;
            chomp $resistance;
            
            print "Now type the voltage of the circuit.\n";
            
            $voltage = <STDIN>;
            chomp $voltage;
            
            goto IF;
        }
        
        my $current = $voltage / $resistance;
        
        my $otherCurrent = $current / 1000;
        
        print "Ohm's law is V = IR, so the current of\n";
        print "the circuit is $current amps,\n";
        print "or $otherCurrent milliAmps.\n";
        
        $calculation = 0;
    }
    elsif($calculation =~ m/resistance/)
    {
        print "Type the current of the circuit.\n";
        print "Add mA at the end of your answer\n";
        print "if your answer is in milliAmps.\n";
        
        my $current = <STDIN>;
        chomp $current;
        
        print "Now type the voltage of the circuit.\n";
        
        my $voltage = <STDIN>;
        chomp $voltage;
        
        IF: if($current && $voltage)
        {
            if($current =~ m/mA/)
            {
                $current =~ s/mA//;
                
                $current /= 1000;
            }
            else
            {
                
            }
        }
        else
        {
            print "Those aren't valid answers.\n";
            print "Please type that again, will ya?\n";
            
            print "Type the current of the circuit.\n";
            print "Add mA at the end of your answer\n";
            print "if your answer is in milliAmps.\n";
            
            $current = <STDIN>;
            chomp $current;
            
            print "Now type the voltage of the circuit.\n";
            
            $voltage = <STDIN>;
            chomp $voltage;
            
            goto IF;
        }
        my $resistance = $voltage / $current;
        
        print "Ohm's law is V = IR, so the resistance of\n";
        print "the circuit is $resistance ohms.\n";
        
        $calculation = 0;
    }
    else
    {
        print "That is not a valid answer.\n";
        print "Retype your answer.\n";
    }
}
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