%out = ();
for (@in){ 
	if (/^([A-Za-z_]+)(\d+)$/){
		# build hash of arrays
		# with alpha part uppercase:original as key
		push @{$out{join ':',uc $1,$1}},$2;
	} else {
		warn "\nInput data error <$_>\n";
	}
}

##</code><code>##

blank_5_str_1
blank_5_str_10
blank_5_str_11
blank_5_str_12
blank_5_str_13
blank_5_str_14
blank_5_str_2
blank_5_str_3
blank_5_str_4
blank_5_str_5
blank_5_str_6
blank_5_str_7
blank_5_str_8
blank_5_str_9

##</code><code>##

blank_5_str_1
blank_5_str_2
blank_5_str_3