Dear monks If I have a function foo(),
sub foo {my $c = shift; $c*=5; return $c;}
All those substitutions works as expected:
$bar =~ s/(\d{2})/joo/msg;
$bar =~ s#(\d{2})#jee#msg;
$bar =~ s#(\d{2})#foo($1)#mesg;
$bar =~ s#(\d{2})#<td>ha ha ha</td>#msg;

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But if want to use all things: a /e modifier, html tags and # instead of / (because \-escaped slashes are not very readable)
$bar =~ s#(\d{2})#<td>foo($1)</td>#mesg;
I only get
Bareword found where operator expected at a.pl line 18, near "<td>foo"
        (Missing operator before foo?)
syntax error at a.pl line 18, near "<td>foo"
Search pattern not terminated at a.pl line 18.

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man perlop is not very helpful.
Any non-whitespace delimiter may replace the slashes. Add space after the "s" when using a character allowed in identifiers. If single quotes are used, no interpretation is done on the replacement string (the "/e" modifier overrides this, however). Note that Perl treats backticks as normal delimiters; the replacement text is not evaluated as a command. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, for example, s(foo)(bar) or s<foo>/bar/. A /e will cause the replacement portion to be treated as a full-fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time. A second "e" modifier will cause the replacement portion to be "eval"ed before being run as a Perl expression.
Tested in Debian Jessie (5.20) and in Debian Buster (5.29).

In reply to Side effects of an /e modifier by timpoiko

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