Decay Rates Perl

#!/usr/bin/perl
#
#
# For calculating decay rates
# tested using Hydrogen Exchange data
#
# Input file format should be:
# <residue #> <value 1> <value 2> ...
# One value for each time in @timeses
#
# Obvi needs Algorithm::CurveFit Module
#
# This is old code no hating
#
# ./hxrates <input file>
#

use Algorithm::CurveFit;

@timeses = (0.01,0.02,0.04,0.075,0.1,0.15,0.175,0.2,0.25 );

$x = $z =0;

open(HXCH, $ARGV[0]);

@hx = <HXCH>;

#print $timeses[0]; die;
foreach $line(@hx) {

   @vols = split(/ /,$line);
   $res = $vols[0];
   shift(@vols);

   foreach $val(@vols) {
      if($val != -10) {
      $val =~ s/\n//;
      $hxvols{$res}->{$x} = $val;

      #print "$hxvols{ $res }{ $x } : $hxvols{ $res }{ $x }{ times }\n";
      $x++;
      $z++;
      }
      else{
      $z++;
      }
   }

 $x=0;
 $z=0;
} #foreach

foreach $key (sort keys %hxvols) {
$a=0;
#for($y=0; $y <= $num; $y++){
foreach $y (sort keys %{ $hxvols{$key} }){

$yval[$a] = $hxvols{$key}{$y};
$xval[$a] = $timeses[$y];
if($yval[$a] != -1){
#print "Y: $yval[$a]\n";
$a++;
}
else{
pop(@yval);
pop(@xval);
}


#print "$key :: $yval[$y] :: $xval[$y]\n";
}

#initialize conditions
@parameters = ( ['n',1,0.00001],['k',10,0.00001] );
#print "$key   @yval :: @xval\n";

#exponential with offset
my $formul = 'n *(1 - exp(-k * x))';
$rate = Algorithm::CurveFit->curve_fit(
    formula => $formul,
        params  => \@parameters,
        variable =>   'x',
        xdata    => \@xval,
        ydata    => \@yval,
        max_iterations => '10000'
);

print "$key $parameters[0][1] $parameters[1][1]\n\n";
}


close(HXCH);