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$element contains "Battery Failure".

There is no such key in %error_code, so $error_code{$element} returns undef.

The nearest valid key would be "SysBatteryFailure" , so you need to figure out how to get that into $element.

ALso, you need to decide if @ILO_ERRORS has a hashref element, or is a simple array. You are attempting to use it both ways.

After you get $element set correctly, if you want to use @ILO_ERRORS as an array, try:

my @ILO_ERRORS = (); # Empty LIST;  not {}, which is a hash ref
...
push @ILO_ERRORS, $error_code{$element};
....
if (@ILO_ERRORS) {
  print "SNMP CRITICAL -  @ILO_ERRORS \n"; # Note - array interpolated
+ INSIDE QUOTES, "\n" added
 ...
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To get $element to work right, you could try:

my ($key) = grep {$error_code{$_} =~/$element/} keys %error_code; 
     # Note - parens around ($key) provide the required list context
$key and push @ILO_ERRORS, $error_code{$key};
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