#!/usr/bin/perl -w
...
use warnings;
...
BEGIN { $^W = 1; }
sub BEGIN {
    if ($^W == 1) {say "Let's get started..."}
    else {die $@}
}

##</code><code>##

my $resistance = <STDIN>;
if ($current =~ /mA/) {
    $current =~ s/mA//;
    $current =~ s/(^\s+| )//;
    $current /= 1000;
}

##</code><code>##

my $input = <STDIN>;
my ($current) = $input =~ /^\s*[\d.]+\s+/ or do {
    error_stuff();
    ... # next, redo, or return, as appropriate
};
$input =~ /\s+mA\s*$/ and $current /= 1000;