I'm not sure what you're trying , but I think this is it

#!/usr/bin/perl --

use strict;
use warnings;

use Test::More qw( no_plan ); 

Main(@ARGV);
exit(0);

sub writeoff {
    my ( $prefix, $last ) = shift()=~ /^(.+)(.)$/;
    $last = $last < 5 ? 0 : 5;
    return "$prefix$last";
}

sub Main {
    my @input = qw( 9.91  9.92  9.95 9.99  );
    my @wanted_output =  qw( 9.90  9.90  9.95 9.95  );
    my @got = map  { writeoff($_) } @input;

    is_deeply( \@got, \@wanted_output, "@wanted_output" );
}
__END__

ok 1 - 9.90 9.90 9.95 9.95
1..1
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