If the input number is 0 or less than 0 than it should jump to some other part say line number 32 of my code. how can I achieve that? any specific function which I can use?

See

See http://perldoc.perl.org/perlintro.html#Writing-subroutines and Modern Perl chapter 5

use strict;
use warnings;
Main( @ARGV );
exit( 0 );
sub Main {
    my $default = 0;
    my $num = Prompt( "Enter num flags:", $default );
    if( $num eq 'purple ){
        PurpleFlags( $num );
    } else {
        BananaFlags( $num );
    }
}
sub PurpleFlags {
   my( $num ) = @_;
   ...
}
sub BananaFlags {
    my( $num ) = @_;
    ...;
}
sub Prompt {
    ExtUtils::MakeMaker::prompt( @_ );
}
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