This is how I would implement the approach suggested by Frankus and others in this thread (though I certainly wouldn't claim to be an experienced Perl programmer). Try this:

#! /usr/local/bin/perl -w

use strict;

# Set up a hash, where $freq{word} = no of occurences of 'word'
# (where word is actually a number in this case)

my %freq;

# Read from filename given on command line, or stdin if no file
# name is given
while (<>)
{
    my @array = split (/\s+/, $_);

    # Update the hash
    foreach (@array) {$freq{$_}++}
}

# Sort the keys of the hash (the words or numbers in the file) into 
# an array in ascending order of $freq{key} (the number of occurences)

my @sorted_array = sort { $freq{$a} <=> $freq{$b} } keys %freq;

# The mode is the last value in the array 
my $mode = pop(@sorted_array);

print "The mode is $mode\n";
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