Use modulo arithmetic.

if ($Number % 5 == 0){
   ...;
}
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Alternatively, as all multiples of five end in a 5 or a 0, you can do something like this:

if ($Number =~ /^-?[0-9]*[05]$/){
   ...;
}
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In both these cases, I'm allowing negative values, like -5 and -10. If you prefer to only allow non-negative values:

if ($Number % 5 == 0 and $Number >= 0){
   ...;
}
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if ($Number =~ /^[0-9]*[05]$/){
   ...;
}
[download]