With the following Perl script loaded in CPerl mode ("emacs -q" then
M-x cperl-mode), when I type [Tab] on the "sub bar" line, this line
is indented instead of remaining at the same place.
----------------------------------------
#!/usr/bin/env perl

sub foo
  {
  }

sub bar
  {
  }
----------------------------------------

In fact, the problem appears more often when one types "sub bar", then
[Enter], then "{". One gets:

sub bar
    {

instead of

sub bar
  {

Another one:

while (<>)
  {
    m:^  \d+ p:
      or die;
    m:^  \d+ :
      or die;
  }

[TAB] on "or die;" gives:
  * a 4-column indentation instead of 6 for the 1st one,
  * a 8-column indentation instead of 6 for the 2nd one.