$- and $= are magically bound to fields in the currently selected IO handle, and can only be integers. Watch:

$ perl -we'$= = "123 abc"; print $='
Argument "123 abc" isn't numeric in scalar assignment at -e line 1.
123
$ perl -we'$= = undef; print $-'
Use of uninitialized value in scalar assignment at -e line 1.
0
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Whatever you feed them is converted to an integer (with a warning, if appropriate) as it is stored. (In the case of $-, it also is made 0 if it was negative.)

One of my all time favorite warning messages:

$ perl -we'undef $=; print $-'
Use of uninitialized value in undef operator at -e line 1.
0
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