Assuming the language consists of 1s and 0s...

sub belongs_to_L {
    my ($s) = @_;
    my ($num_zeros,$num_ones,$junk);

   ($num_zeros,$num_ones,$junk) = ( $s =~ m/^(0*)(1*)(.*)/);
   
   return 1 if ( length($zeros) == length($ones) and 
                 length($junk)  == 0);
   return 0;
}
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