ken@titan ~/tmp/pm_11152739_file_rename
$ ls -1
'test 4_5 6'
test_1_2_3
'test_a b_c d_e f'

$ perl -e '
    use strict;
    use warnings;
    use File::Rename "rename";
    my @files = (
        "test 4_5 6",
        "test_1_2_3",
        "test_a b_c d_e f",
    );
    rename \@files, {
        _code => sub { s/_/ /g; },
        verbose => 1,
        no_action => 1,
    };
'
rename(test 4_5 6, test 4 5 6)
rename(test_1_2_3, test 1 2 3)
rename(test_a b_c d_e f, test a b c d e f)

$ ls -1
'test 4_5 6'
test_1_2_3
'test_a b_c d_e f'

$ perl -e '
    use strict;
    use warnings;
    use File::Rename "rename";
    my @files = (
        "test 4_5 6",
        "test_1_2_3",
        "test_a b_c d_e f",
    );
    rename \@files, {
        _code => sub { s/_/ /g; },
        verbose => 1,
    };
'
test 4_5 6 renamed as test 4 5 6
test_1_2_3 renamed as test 1 2 3
test_a b_c d_e f renamed as test a b c d e f

$ ls -1
'test 1 2 3'
'test 4 5 6'
'test a b c d e f'