... you can get accurate calculations by adding a use bignum; to your code

I think this is the only solution that requires no modification to the original line of code:
for ($x=0.8; $x > 0.1; $x -= 0.01) { print "$x\n"; }

It's probably worth pointing out to the OP that it works simply because bignum uses decimal (base 10) arithmetic.

There are other ways to force base 10 arithmetic - eg Math::Decimal, Math::Decimal64 and Math::Decimal128. (The last 2 are a plug, and will work far more quickly than bignum.)
However, they would all require some changes to the given line of code. For example:

use Math::Decimal64;
for ($x = Math::Decimal64->new('0.8'); $x > '0.1'; $x -= '0.01') { pri
+nt "$x\n"; }
# or if one is insistent upon receiving 0.xx formatting of the values 
+(instead of "xxe-2") formatting:
for ($x = Math::Decimal64->new('0.8'); $x > '0.1'; $x -= '0.01') { pri
+nt "$x" + 0, "\n"; }
[download]

Cheers,
Rob