while(<>){                # <>: used handle is STDIN
   print if $.==5;        # so $. is that handle's line number
}

##</code><code>##

my $file="/etc/hosts";
my $i=0; # let's use 1-based numbers
open(FH,"<",$FILE) or die "dying on $FILE";
while(<FH>){
   $i++;
   print "$i: $_" if $i == 5;
}