#!/usr/bin/perl

$num{'15'}=1;
$num{'20'}=2;
$num{'25'}=3;
$num{'30'}=4;

$x=shift;  # take value from @ARGV

print value_sought( $x ), $/;

sub value_sought
{
    my $x = shift;  # take value from @_
    foreach my $k ( sort { $a <=> $b } keys %num )
    {
        return $num{$k} if ( $x < $k );
    }
    return "$x is larger than available num keys";
}