Always check whether your regular expression matched before accessing the capture variables $1, $2 ...:

#!/usr/contrib/bin/perl

# Parses a string in the format:
# REFDATA_ERROR[type=<1>,system=<2>,category=<3>,code=<4>]
# Prints out the four parameters, <1>, <2>, <3>, <4>
sub parse_error
{
  $params =~ m/REFDATA_ERROR\[type=(.*),system=(.*),category=(.*),code
+=(.*)\]/
    or die "The input string was invalid: '$params'";
  print "$params\n";
  print "$1\n";
  print "$2\n";
  print "$3\n";
  print "$4\n";
};

# Main
parse_error("REFDATA_ERROR[type=val1,system=val2,category=val3,code=va
+l4]");
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If you had used strict, Perl would have told you that your variable $params is undefined, as you seem to have a misconception on how parameters are passed around in Perl. The input parameters to your function live in the @_ array. Also, you are using a prototype on your sub declaration, which is in most cases a bad thing to do unless you know what you're doing (you don't).