Read the docs for Net::POP3. get returns a reference to an array of lines, not the whole message.

get ( MSGNUM [, FH ] )

    Get the message MSGNUM from the remote mailbox. If FH is not given
+ then get returns a reference to an array which contains the lines of
+ text read from the server. If FH is given then the lines returned fr
+om the server are printed to the filehandle FH.
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You just need to search through those lines for the subject.

my $msg = $pop->get($msgnum);
for my $line ( @$msg ) {
    if( $line =~ /^Subject:(.*)/ ) {
        print "Subject is $1";
        last;
    }
}
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Or, similarly, use print grep { /^Subject:/ } @$msg;

Update: I should add that when I first read the subject "Problem with $1", my first thought was, "It's not $1,000,000?"

Update: grep doesn't terminate after the first match. List::Util and first is better.