Not true. Using -Mre=debug (output trimmed a bit)
/a|b/ yields:

Compiling REx `a|b'
   1: BRANCH(4)
   2:   EXACT <a>(7)
   4: BRANCH(7)
   5:   EXACT <b>(7)
   7: END(0)
minlen 1
[download]

but /a\x7cb/ yields:

Compiling REx `a\x7cb'
   1: EXACT <a|b>(3)
   3: END(0)
anchored `a|b' at 0 (checking anchored isall) minlen 3
[download]

(tested w/ 5.005_03, 5.6.0, and 5.8.0)