As the benchmarks show, your algorithm, is a good one. With a few tweaks to the implementation it runs faster still:

sub top_x {
    my( $n, $aref ) = @_;
    my @topN = (0)x$n--;
    for my $val ( @$aref ) {
        next if $topN[ $n ] > $val;
        $topN[ $_ ] < $val 
            and splice( @topN, $_, 0, $val ), last 
            for 0 .. $n;
    }
    return @topN[ 0 .. $n ];
}
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If this is more than an intellectual exercise, and you can handle Inline::C, then the same algorithm C-ified really flies:

Update: Correct the Inline::C implementation below to avoid calloc and allow me to free the temporary C array.

void topN( int n, AV*data ) {
    int *topN;
    int len = av_len( data );
    int i, j, k;

    Inline_Stack_Vars;
    Newz( 1, topN, n + 1, int );
    
    for( i = 0; i <= len; i++ )    {
        int val  = SvIV( *av_fetch( data, i, 0 ) );
        for( j = 0; j < n; j++ ) {
            if( topN[ j ] > val ) continue;
            if( topN[ j ] < val ) {
                for( k = n; k > j; k-- ) topN[ k ] = topN[ k-1 ];
                topN[ j ] = val;
                break;
            }
        }
    }
    
    Inline_Stack_Reset;
    for( i = 0; i < n; i++ )
        Inline_Stack_Push( sv_2mortal( newSViv( topN[ i ] ) ) );
    
    Safefree( topN );
    Inline_Stack_Done;
}
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