The easiest is probably a goto:

TOP:
my $num = <STDIN>;
if( $num != 10 and $num != 0 ) #ok, a horrible condition
{
  goto TOP;
}

#rest of code
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Other ways would involve possibly a subroutine or hacking about with a while loop:

my $input;
while( $input != "good" )
{
  print "Please enter number:\n";
  $input = <STDIN>;
}

#rest of code
[download]