use strict;
use warnings;
use 5.016;

my $fname = 'data.txt';

open my $INFILE, '<', $fname
    or die "Couldn't open $fname: $!";

my $file;

{
    local $/ = undef;   #No line terminator, so a line
                        #is everyting in the file
    $file = <$INFILE>;  #Read one line.
}

# $/ is restored to its default value of "\n" here

close $INFILE
    or die "Couldn't close $fname: $!";