At first sight it looked as if a minor modification was necessary, but grep has run through the array and returned a list before the lhs can get at each match variable, preventing them being assigned to keys of %inp (normally you'd use $1 not $_).

map was probably invented for this kind of reason. But the example doesn't really demand it that strongly when the lhs is a hash - 'for' still seems adequate and simple to me:

#!/usr/bin/perl 

use strict ;
use warnings ;

my @inp = ("abc_A_bla", "abc_B_bla", "abc_C_bla") ;

my %inp ;

$inp{ $1 } = /\w{3}_(\w)/ || next for @inp; 

foreach ( keys %inp ) {
    print "Found $_\n" ;
}
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