You already have the keys to match, so why not match them? Then if it doesn't match, have a backup s///. (Also implementing graff's suggestion).

my $keys = join '|', keys %inst;
my $prefix = 'something';
my $suffix = 'else';
s/(^.*<)\:($keys)>/$1\e[31m$2\e[0m>/
  or
s/(^.*<)\:([a-z]+)>/$1$prefix$2$suffix>/
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