I presume(perhaps wrongly) that you mean something like the following:

#!/usr/bin/perl -w

use strict;
use warnings;

sub mean {
    my (@data) = @_;
    my $sum;
    foreach (@data) {
        $sum += $_;
    }
    return ( $sum / @data );
}
sub median {
    my (@data) = sort { $a <=> $b } @_;
    if ( scalar(@data) % 2 ) {
        return ( $data[ @data / 2 ] );
    } else {
        my ( $upper, $lower );
        $lower = $data[ @data / 2 ];
        $upper = $data[ @data / 2 - 1 ];
        return ( mean( $lower, $upper ) );
    }
}
sub std_dev {
    my (@data) = @_;
    my ( $sq_dev_sum, $avg ) = ( 0, 0 );

    $avg = mean(@data);
    foreach my $elem (@data) {
        $sq_dev_sum += ( $avg - $elem )**2;
    }
    return ( sqrt( $sq_dev_sum / ( @data - 1 ) ) );
}

my ( $data, @dataset,%h,$lc );
my $file = "fake.txt";
open(IN, "<", $file) or die "can't open $file $!\n";
while (<IN>) {
    chomp;
    $lc++;
    $h{$lc} = $_;
}
close IN;

#while (<DATA>) {
#    chomp;
#    $lc++;
#    $h{$lc} = $_;
#}
#@dataset = (values %h);

print "vals " .scalar(keys %h) . "\n";
print "Median: ",        median(@dataset),  "\n";
print "Mean: ",          mean(@dataset),    "\n";
print "Standard Dev.: ", std_dev(@dataset), "\n";


__DATA__
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[download]

If this assignment actually requires you to process the data in the subs by handing in the hash instead of the array, you should find enough here to get you started... . Run it with 'perl -d' to see what things look like. The debugger is easy to use.

One also has to presume that 'fake.txt' looks like __DATA__, which you did not specify.