Yes,
if ( $var_a =~ /^abcde$/ && $var_b =~ /^abcde$/ && ....
is not equivalent to
if (q{abcde} =~ /^(?=$var_a$)(?=$var_b$)(?=$var_c$)/)
but it is equivalent to
if (q{abcde} =~ /^(?=\Q$var_a\E$)(?=\Q$var_b\E$)(?=\Q$var_c\E$)/)
Of course, that only works because abcde meets the following conditions:

• abcde doesn't contain any characters with special meanings to regexps.
• You're matching the entire string.

In other words, your "optimization", aside from being hard to read and probably being slower, has very limited usage.

It would be better to use:

my $match = 1;
foreach ($var_a, $var_b, $var_c) {
   next if /abcdefgh/;
   $match = 0;
   last;
}

if ($match) {
   ...
}
[download]

or

if ((grep /abcdefgh/, $var_a, $var_b, $var_c) == 3) {
   ...
}
[download]

or

use List::MoreUtils qw( all );

if (all { /abcdefgh/ } $var_a, $var_b, $var_c) {
   ...
}
[download]

The grep solution is easier to read than the foreach solution, but it has the disadvantage of always executing the regexp for every item in the list. The foreach and the all solutions execute the regexp as few times as possible

(Pardon the lateness of my reply; I was on vacation.)