The phrase "perfect substitute" means that two things are exactly the same and can be used in place of each other without any difference. Full definition
The general ideological proximity of progressive parties doesn't mean they are seen as perfect substitutes by swing voters. (theconversation.com)
Perfect substitute if you want to impress your guests during a festive party. (yurielkaim.com)
We show that when SGE is a less perfect substitute for abatement — that is, when damages from carbon concentrations matter more relative to damages from temperature — then SGE is used less intensively and abatement is used more intensively. (ceassessment.org)