First, you may want to look at a module like HTML::TextToHTML to take care of this particular type of example.

But for a general case, it would be something like this (the sort is to prevent short matches like f from usurping longer ones like foo)

my %repls = qw(
    <   &lt;
    >   &gt;
    foo bar
    bar foo
    f   baz
);
my $replstr = join '|', sort {length $b <=> length $a} keys %repls;
s/$replstr/$repls{$&}/g;
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Update: Shamelessly stole Abigail's initialization syntax