Another example using a loop:

#!/usr/bin/env perl

use strict;
use warnings;
use feature "say";

my $radius;

for(;;)
{
    say "Enter radius:";
    $radius = <stdin>;
    last if $radius >= 0 ;
    say "error : radius must be >= 0";
}
                                                             
say "circumference: ", 2*3.14*$radius;
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