I'll second Joost's approach, but make a few suggestions for readability / maintainability.

• Use qw so to de-clutter the list of strings.
• Use an explicit variable in the for; they are cheap and they make your intention clear.
• Use [ ] braces for the the regex separator so you won't have to backslash the slash. This de-emphasizes some of the executable line noise effect
• Use the regex x modifier to put some whitespace and comments in here.

foreach my field (qw(postby title teaser content)){
  $in{$field} =~ s[ (\r? \n){2,} ]          # two or more CR
                  [ \n </p> \n <p> \n]gx;   # Close one para, open ano
+ther
}
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throop