It doesn't involve a regex, which would seem inappropriate for this problem, but either of the following two approaches should work:

>perl -wMstrict -le
"my @raw_files = qw{ 12.png 06.png 06.png 03.png 07.png 07.png };
 my $blank_png = 'blank.png';
 my @blanked_files = interleave_blanks($blank_png, @raw_files);
 print qq{@blanked_files};
 sub interleave_blanks {
     my $blank_file = shift;
     my $prev_file = '';
     map {
         my $repeat = $_ eq $prev_file;
         $prev_file = $_;
         $repeat ? ($blank_file, $_) : ($_);
         }
         @_
         ;
     }
"
12.png 06.png blank.png 06.png 03.png 07.png blank.png 07.png

>perl -wMstrict -le
"my @raw_files = qw{ 12.png 06.png 06.png 03.png 07.png 07.png };
 my $blank_png = 'blank.png';
 my @blanked_files = do {
     my $prev_file = '';
     map {
         my $repeat = $_ eq $prev_file;
         $prev_file = $_;
         $repeat ? ($blank_png, $_) : ($_);
         }
         @raw_files
         ;
     };
 print qq{@blanked_files};
"
12.png 06.png blank.png 06.png 03.png 07.png blank.png 07.png
[download]

Update:

In case the animation sequence is a continuous loop and thus requires a blank .png inserted after the last .png in the array if the last .png is the same as the first .png, this should do the trick (although it might be a little more elegant without a bunch of reverses scattered all over the place):

>perl -wMstrict -le
"my $blank_png = 'blank.png';
 print qq{\noutput:};
 for (@ARGV) {
     print qq{ in: $_};
     my @raw_files = split;
     my @blanked_files = interleave_blanks($blank_png, @raw_files);
     print qq{out: @blanked_files};
     }
 sub interleave_blanks {
     my $blank_file = shift;
     my $prev_file  = $_[0];
     reverse map {
         my $repeat = $_ eq $prev_file;
         $prev_file = $_;
         $repeat ? ($blank_file, $_) : ($_);
         }
         reverse @_
         ;
     }
"
"12.png 06.png 06.png 03.png 07.png 07.png"
"07.png 12.png 06.png 06.png 03.png 07.png 07.png"

output:
 in: 12.png 06.png 06.png 03.png 07.png 07.png
out: 12.png 06.png blank.png 06.png 03.png 07.png blank.png 07.png
 in: 07.png 12.png 06.png 06.png 03.png 07.png 07.png
out: 07.png 12.png 06.png blank.png 06.png 03.png 07.png blank.png 07.
+png blank.png
[download]

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