Tested code:

$ cat 11104497.pl
#!usr/bin/local/perl
use strict;
use warnings;

my %count;
my $file_name = shift or die "Usage: perl $0 [FILE]\n";

open my $fh, '<', $file_name or die "Could not open '$file_name' $!";

while ( my $line = <$fh> ) {
    chomp $line;
    foreach my $word ( split /\s+/, $line ) {
        $count{$word}++;
    }
}

foreach my $word ( sort keys %count ) {
    printf "%-31s %s\n", $word, $count{$word};
}

# ..

my @sorted_by_count = sort { $count{$b} <=> $count{$a} } keys %count;

print "These are the 10 most frequented words of $file_name :\n";

print "$_ occured $count{$_} times\n" for @sorted_by_count[ 0 .. 9 ];
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Output:

$ cat janne.txt
This is a test.
This is only a test.
Had this been an actual emergency, the attention signal you just heard
+ would have been followed by news or instructions.
This has been a test.
$
$ perl 11104497.pl janne.txt
Had                             1
This                            3
a                               3
actual                          1
an                              1
attention                       1
been                            3
by                              1
emergency,                      1
followed                        1
has                             1
have                            1
heard                           1
instructions.                   1
is                              2
just                            1
news                            1
only                            1
or                              1
signal                          1
test.                           3
the                             1
this                            1
would                           1
you                             1
These are the 10 most frequented words of janne.txt :
a occured 3 times
This occured 3 times
been occured 3 times
test. occured 3 times
is occured 2 times
has occured 1 times
or occured 1 times
news occured 1 times
by occured 1 times
this occured 1 times

$
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