Its a global substitution, so even if it was s/FOO/BAR/g there would be no need to repeat that, the global substitution took care of it in one pass

Not necessarily:

$ perl -E '$_ = "caattt"; s/at//g; say'
catt
$ perl -E '$_ = "caattt"; 1 while s/at//g; say'
ct
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Without knowing FOO and BAR, we can't say whether new instances of FOO might be created in the string when the substitution is performed.