while (<STDIN>) {
    chomp              # same as chomp $_
    if (/^#/) {....}   # same as if ($_ =~ /^#/) { ....}
  }

##</code><code>##

  while (my $line = <STDIN>)
  {
    chomp $line;             # same as chomp $_
    if ( $line =~ m/^#/ )
    {
      # Action on $line.
    }
  }

##</code><code>##

$line =~ s/^#/# Comment /;