Originally (i.e. prior to running the earlier tests) I did not have a ~/.perldb file. After posting, I was doing some cleanup and putting things back to the way they were. I did some checking. Curiously, regardless of whether that file was zero-length:

  DB<2> c
Here is ~/.perldb:


main::(./foo.pl:10):    my $foo = 1; # set breakpoint here and issue p
+ @xx
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or removed completely:

  DB<2> c
Here is ~/.perldb:

cat: /home/ken/.perldb: No such file or directory

main::(./foo.pl:10):    my $foo = 1; # set breakpoint here and issue p
+ @xx
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The coloured and pretty-printed output persisted:

  DB<2> p @xx
abcdeARRAY(0xa004294d8)
  DB<3> c
Show result of p @xx when issued from within program
[
    [0] "a",
    [1] "b",
    [2] "c",
    [3] "d",
    [4] "e",
    [5] [
            [0] 1,
            [1] 2,
            [2] 3
        ]
]
Debugged program terminated.  Use q to quit or R to restart,
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This is the same output as I get with:

$ perl -E '
    use Data::Printer;
    my @x = (qw{a b c d e}, [1,2,3]);
    say @x;
    p @x;
'
abcdeARRAY(0xa00003bc8)
[
    [0] "a",
    [1] "b",
    [2] "c",
    [3] "d",
    [4] "e",
    [5] [
            [0] 1,
            [1] 2,
            [2] 3
        ]
]
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For comparison, neither of these outputs are coloured:

$ perl -E '
    use Data::Dumper;
    my @x = (qw{a b c d e}, [1,2,3]);
    say @x;
    print Dumper \@x;
'
abcdeARRAY(0xa00003bc8)
$VAR1 = [
          'a',
          'b',
          'c',
          'd',
          'e',
          [
            1,
            2,
            3
          ]
        ];
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$ perl -E '
    use Data::Dump;
    my @x = (qw{a b c d e}, [1,2,3]);
    say @x;
    dd \@x;
'
abcdeARRAY(0xa00003bc8)
["a" .. "e", [1, 2, 3]]
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I don't know why the Data::Printer formatting is persisting in the debug output. Investigating why this happens may shed some light on the "prints twice" problem.