$ perl -Mre=debug -e " $x = q{\d}; $f = 1234; $f =~ q{$x} "
Compiling REx "$x"
Final program:
   1: EOL (2)
   2: EXACT <x> (4)
   4: END (0)
anchored "x" at 0 (checking anchored) minlen 1
Guessing start of match in sv for REx "$x" against "1234"
Did not find anchored substr "x"...
Match rejected by optimizer
Freeing REx: "$x"

$ perl -Mre=debug -e " $x = q{\d}; $f = 1234; $f =~ qq{$x} "
Compiling REx "\d"
Final program:
   1: DIGIT (2)
   2: END (0)
stclass DIGIT minlen 1
Matching REx "\d" against "1234"
Matching stclass DIGIT against "1234" (4 bytes)
   0 <> <1234>               |  1:DIGIT(2)
   1 <1> <234>               |  2:END(0)
Match successful!
Freeing REx: "\d"

$ perl -Mre=debug -e " $x = q{\d}; $f = 1234; $f =~ qq{\$x} "
Compiling REx "$x"
Final program:
   1: EOL (2)
   2: EXACT <x> (4)
   4: END (0)
anchored "x" at 0 (checking anchored) minlen 1
Guessing start of match in sv for REx "$x" against "1234"
Did not find anchored substr "x"...
Match rejected by optimizer
Freeing REx: "$x"

$ perl -Mre=debug -e " $x = q{\d}; $f = 1234; $f =~ q{\$x} "
Compiling REx "\$x"
Final program:
   1: EXACT <$x> (3)
   3: END (0)
anchored "$x" at 0 (checking anchored isall) minlen 2
Guessing start of match in sv for REx "\$x" against "1234"
Did not find anchored substr "$x"...
Match rejected by optimizer
Freeing REx: "\$x"