s/(?<=^| )blah(?= |$)/something/g;

I may be wrong but I thought look-behinds were fixed-width so the following gives a compilation error.

$ perl -le '
> $str = q{adfsdheHjdGafefaffJaff};
> $str =~ s{(?<=^|[A-Z])a}{999}g;
> print $str;'
Variable length lookbehind not implemented in regex; marked by <-- HER
+E in m/(?<=^|[A-Z])a <-- HERE / at -e line 3.
$
[download]

You could use an alternation of two look-behinds.

$ perl -le '
> $str = q{adfsdheHjdGafefaffJaff};
> $str =~ s{(?:(?<=^)|(?<=[A-Z]))a}{999}g;
> print $str;'
999dfsdheHjdG999fefaffJ999ff
$
[download]

I hope this is of interest.

Cheers,

JohnGG