#!/usr/bin/perl

use strict;
use warnings;

die "Usage: $0  input.file\n" unless ( @ARGV == 1 and -f $ARGV[0] );

my $input_file = shift;
my $input_byte_count = -s $input_file;
{
    local $/;  # set input_record_separator to undef -- switch to "slurp" mode
    open( I, $input_file ) or die "$input_file: $!";
    my $whole_file = <I>;
    close I;
}
if ( length( $whole_file ) < $input_byte_count ) {
    die "$0:  Perl can't read all of $input_file";
}
elsif ( length( $whole_file ) > $input_byte_count ) {
    die "$0: Either -s $input_file is lying, or bytes were added during read\n";
}
else {
    warn "$0: got exactly $input_byte_count bytes from $input_file\n";
}

# now, what was it you need to do with $whole_file? ...

# to normalize all whitespace to " " (making it just one long line):
s/\s+/ /g;
print;