As suggested above, here is a slightly modified version of your script with addition of just 2 lines.

use warnings;
use strict;
print "please enter name of file: ";
chomp($ARGV=<>);
open (FILE, $ARGV) or die "Can't open '$ARGV': $!";
my $lines;
$lines++ while(<FILE>);

print "\$lines: $lines";
my $last;
seek FILE,0,0;
my $first = <FILE>;
while (<FILE>) { $last = $_ }
close FILE;
if($lines<5){
print "file must have more than 5 lines";
}
else{

print "\$first: $first";
print "\$last: $last";
}
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