Just my 2 cents, I'm sure it can be improved a lot. I simply found that is more readable like this... and at least is a little more compact.

#!/usr/bin/perl -w

use strict;
use warnings;
use feature qw(say);
use English qw(-no_match_vars);

BEGIN { $^W = 1; }
sub BEGIN {
    if ($^W == 1) {say "Let's get started..."}
    else {die $@}
}

local $OUTPUT_AUTOFLUSH = 1;
local $OUTPUT_RECORD_SEPARATOR = *\ ;

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

my $type_current = <<NOTE_TWO;
      Type the current of the circuit.
      Add mA at the end of your answer,
      if your answer is in milliAmps:
NOTE_TWO

my $type_voltage = "Now type the voltage of the circuit:";
my $type_resistance = "Type the resistance of the circuit:";
my $ohmlaw = "Ohm's law is V = IR, so ";

my $i = 0;
while ($i == 0) {
    my $calculation = <STDIN>;
    if ($calculation =~ /voltage/) {
        say $type_current;
        my $current = <STDIN>;
        say $type_resistance;
        my $resistance = <STDIN>;
          if ($current =~ /mA/) {
              $current =~ s/mA//;
              $current =~ s/(^\s+| )//;
              $current /= 1000;
        }
          else {()}
        say $ohmlaw . "the voltage of\nthe circuit is " . $current * $
+resistance . " volts.";
        ++$i;
      }
    elsif ($calculation =~ /current/) {
        say $type_resistance;
        my $resistance = <STDIN>;
        say $type_voltage;
        my $voltage = <STDIN>;
        say $ohmlaw . "the current of\nthe circuit is ". $voltage / $r
+esistance . " amps., or " . $voltage /($resistance*1000) . " milliAmp
+s.";
        ++$i;
    }
    elsif ($calculation =~ /resistance/) {
        say $type_current;
        my $current = <STDIN>;
        say $type_voltage;
        my $voltage = <STDIN>;
        if ($current =~ /mA/) {
             $current =~ s/mA//;
             $current =~ s/(^\s+| )//;
             $current /= 1000;
        }
        else {()}
        say $ohmlaw ."the resistance of\nthe circuit is ". $voltage / 
+$current . "ohms.";
        ++$i;
    }
    else {say "That is not a valid answer.\nRetype your answer."}
}
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