i wrote a script which measure the hydrogen bond in RNA molecule, but i have this error, which i cannot handle. please help me. use of uninitialized value line 27.

#!/usr/bin/perl
use warnings;
use strict;
my @structure;
my @bases;
my $basestring = <STDIN>;
chomp($basestring);
my $parenstring = <STDIN>;
chomp($parenstring);
print evalRnaStructure($basestring,$parenstring)," hydrogen bonds in t
+his 

structure.\n";
sub evalRnaStructure {
my ($basestring,$structurestring) = @_; 
@bases = split(//,"5.$basestring.3"); 
@structure = split(//,"($structurestring)"); 
return evalRna(0,$#structure); 
}
sub evalRna {
my ($l,$r) = @_; 
my %bonds= (GU=>1,UG=>1,AU=>2,UA=>2,CG=>3,GC=>3); 
my $numBonds = $bonds{$bases[$l].$bases[$r]}; 
my $level = 0; 
my $ii = $l; 
for (my $i=$l+1; $i<=$r; $i++) { 
$level--  if ($structure[$i] eq ")"); 
if ($level==0) { 
$numBonds+= evalRna($ii,$i)  if ($structure[$i] eq ")");
$ii = $i; 
}
$level++ if ($structure[$i] eq "("); 
}
return $numBonds; 
}

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In reply to recursive algorithm by Anonymous Monk

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