map works... but there is a feeling that it is not needed. If I do it, I will simply:

use strict;
use warnings;
use Data::Dumper;

my $AoH_orig = [ { 'title' => 'aaa-aaa',
                   'name'  => 'tom' },
                 { 'title' => 'bbb-bbb',
                   'name'  => 'kathy' },
                 { 'title' => 'ccc-ccc',
                   'name'  => 'bill' } 
                 ];

for (@$AoH_orig) {$_->{'title'} =~ s/-/_/g};

print Dumper($AoH_orig);
[download]

Which gives you what you wanted:

$VAR1 = [
          {
            'name' => 'tom',
            'title' => 'aaa_aaa'
          },
          {
            'name' => 'kathy',
            'title' => 'bbb_bbb'
          },
          {
            'name' => 'bill',
            'title' => 'ccc_ccc'
          }
        ];
[download]