#!/usr/bin/perl

use strict;
use warnings;

my $infile = shift;

open my $INPUT_FILE, "<", $infile" or die "can't open $infile: $!";

local $/; # the whole file will be slurped, even if it has several lines
my $dna = <$INPUT_FILE>;         
if ( $dna  =~ m/[^actg\s]/i ) {
    print "File contains something besides actg sequence.\n";
} else {
    print "good!\n";
}            
close $INPUT_FILE;