Another way is to use the four argument form of substr rather than concatenation.

$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
1234
5678
1000
1233
123456
EOD

say for
   map {
       chomp;
       substr $_, length, 0, substr $_, 0, 2;
       $_;
   } <$inFH>;'
123412
567856
100010
123312
12345612
$
[download]

I hope this is helpful.