This is a theoretically slightly more efficient version, but on a 10e6 value/10 unique dataset, the difference is negligible.

#! perl -slw
use strict;

my %hash;
++$hash{ <> } until eof();

my @top5;
push @top5, sort{ $hash{ $b } <=> $hash{ $a } } map scalar each %hash,
+ 1 .. 5;

while( my $next = each %hash ) {
    for my $i ( reverse 0 .. 4 ) {
        if( !defined( $top5[ $i ] ) or $hash{ $next } > $hash{ $top5[ 
+$i ] } ) {
            splice @top5, $i, 0, $next;
            pop  @top5 if @top5 > 5;;
            last;
        }
    }
}
chomp and print for @top5;
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