True, but this doesn't apply here, it's literal a search pattern (i.e. compile time), the m is just optional:

Both generate exactly the same opcodes:

lanx@nc10-ubuntu:~$ perl -MO=Concise 
'abc' =~ '\w{3}'
__DATA__
5  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -:1) v:{ ->3
4     </> match(/"\\w{3}"/) vKS/RTIME ->5
3        <$> const[PV "abc"] s ->4
- syntax OK
lanx@nc10-ubuntu:~$ perl -MO=Concise 
'abc' =~ m'\w{3}'
__DATA__
5  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -:1) v:{ ->3
4     </> match(/"\\w{3}"/) vKS/RTIME ->5
3        <$> const[PV "abc"] s ->4
- syntax OK
[download]

"expressions evaluated at runtime" are things like variables or functions or do-blocks...

  DB<103> $x= '\w{3}'
 => "\\w{3}"

  DB<104> 'abc' =~ $x
 => 1

  DB<105> sub regex { '\w{3}' }

  DB<106> 'abc' =~ regex()
 => 1
[download]

OK, I see the light now.

<pedantic>
There's m//, and =~. m// can be used without =~ to match against $_, or with =~ to match against something else.

Without the presence of =~, m// can omit the m if the delimiter is //, else it is required.

=~ expects to see a match, substitution, or translation on the RHS, and assumes a match without m//, s//, y//, or tr//.
</pedantic>

And since m//, and =~ are 2 separate entities (that slightly overlap), the documentation doesn't tie these together explicitly (IMHO).

-QM
--
Quantum Mechanics: The dreams stuff is made of