Use a hash:

my %subs = (
    tag1 => 'subst1',
    tag2 => 'subst2',
    ...
);

my $pattern = join('|', keys %subs);
$str =~ s/($pattern)/$subs{$1}/g;
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It would be a good idea to simplify $pattern if there is some common pattern to the tags.

By the way, you need to use the g modifier to replace all occurrences of each tag if it can occur more than once in the string, also if you are substituting each tag separately.