$filename = $ARGV[0]; #corrected from @ARGV[0] as suggested by  eyepopslikeamosquito
open (FILE,"$filename") || die "Can't Open $filename: $!\n";
@LINES=<FILE>;   #slurping into array

foreach my $line(@LINES){ 
    #process something here
 }

##</code><code>##

$filename = $ARGV[0];
open (FILE,"$filename") || die "Can't Open $filename: $!\n";

while<FILE>{ #dealing with it per-lines
  #process something here
}