The second part of a substitution is just a plain string, not a regular expression. To include a part from the matched string, you have to use capture groups, introduced by parentheses. To uppercase a part of a string, use \U: 
$this_term =~ s/\.([a-z])/.\U$1/g;

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In reply to Re: replacing part of a string with its uppercase via regex by choroba
in thread replacing part of a string with its uppercase via regex by brewzzer

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