Here is another interpration of the OP's requirements :
wanting to get an array ref containing ONLY the "title" element values, with the "_" substitution:

my $Aref_new =[ map {(my $x=$_->{'title'}) 
                 =~ s/ /_/g;$x } @$AoH_orig]; 
print Dumper(\$Aref_new);
[download]

Note: Using the temporary "$x" allows the use of the "map" while preserving the original $AoH_orig, and getting the array ref (Hence the square brackets) containing the modified values.