A slightly different approach.

#! perl -slw
use strict;

my $g = 
qq[PPBRB
PRBBB
BRRRB
BRRRB
BGGBB];

my $w = 5 -1;
my( $o1, $o2 )= ( $w * 2, $w *2 +1 );

$g =~ s[\n]["\n" . '-+' x $w . "-\n"]eg;
$g =~ s[([PRGB])(?=[PRGB])][$1|]g;

$g =~ s[(?<=([PRGB]))\|(?=\1)]            [ ]g;
$g =~ s[(?<=(.).{$o2})[-](?=.{$o2}\1)]    [ ]gs;
$g =~ s[(?<= .{$o1}[-])\+(?=[-].{$o1} )]  [-]gs;
$g =~ s[(?<= .{$o1}[ -])\+(?=[ -].{$o1} )][ ]gs;
$g =~ s[(?<=\|.{$o1} )\+(?= .{$o1}\|)]    [|]gs;

print $g;
__END__
P:\test>345780
P P|B|R|B
 +-+ +-+
P|R|B B B
-+ +---+
B|R R R|B
 |     |
B|R R R|B
 +---+-+
B|G G|B B
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