LINE:
while (<>) {
  ...             # your program goes here
}

##</code><code>##

LINE: while (defined($_ = <ARGV>)) {
    $first ||= $_;
    $last = $_;
    ++$count;
}
{
    die "ERROR found $count lines. Need >5" unless $count > 5;
    print "FIRST:$first\nLAST:$last\n";
}

##</code><code>##

perl -ne '$first ||=$_; $last=$_; $count++}{die qq(ERROR found $count lines. Need >5) unless $count>5; print qq(FIRST:$first\nLAST:$last\n)' YOUR-FILE-NAME-here.txt