It's not homework is it?

No need to use a regex:

my @output = split(/\n/, `set`);
my %environment;
foreach (@output) {
    my ($key, $value)= split /=/;
    $environment{$key}=$value;
}
foreach $variable qw/USERNAME HOMEDRIVE/ {
    ${$variable}=$environment{$variable};
}
print "my name is $USERNAME
my home is at $HOMEDRIVE";
[download]

Output:

my name is KJM
my home is at C:
[download]

I had to adapt the environment variables a bit as I'm on Windows XP.

Update: I should have used the %ENV-hash, but TIMTOWTDI!