If you're satisfied to limit yourself to just Linux you should be able to get away rather simply with parsing the output from ifconfig -a and looking for just those interfaces matching /^eth\d+/.

open( IFCFG, "ifconfig -a |" ) or die "Can't open pipe from ifconfig: 
+$!\n";
my %ifs;
while( <IFCFG> ) {
  chomp;
  ## ...
}
close( IFCFG );
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