use strict;
use warnings;

use Test::More qw/no_plan/;

my @interval = (
    [  0,         '' ],
    [  5,   'orange' ],
    [ 10,     'grey' ],
    [ 15,     'blue' ],
);

sub interval {
    my $x = shift;

    # The special case 0 should be threated separately
    my $counter = 1;
    while( $counter <= $#interval && $x > $interval[ $counter ][ 0 ] )
+ {
        ++$counter;
    }
    return $interval[ $counter ][ 1 ];
}

ok( interval( 3 )  eq 'orange', "0 < 3 <= 5" ); 
ok( interval( 7 )  eq 'grey',   "5 < 7 <= 10" ); 
ok( interval( 10 ) eq 'grey',   "5 < 10 <= 10" );
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Update: Oops. Wrong order of clauses in the while condition. Now fixed.