So @context contains search strings? (This is why it's important to give a runable piece of code, including sample input. It takes the guessing out of the game.)

my @contexts = qw(
   demo-outgoing
   demo
);

local $_ = 'foo demo-outgoing bar';


my $regexp = join('|', map { quotemeta($_) } @contexts);
s/($regexp)/__user__${1}__/g;


print("$_\n");
# foo __user__demo-outgoing__ bar
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