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s/\@(.*?)\@/ ... /g;

s<\@(.*?)\@>{ ... }ge;

while(<>) {
    s/\@(.*?)\@/ subst($1) /ge;
...
    $s =~ s/([a-eA-C])/$r{$1}/g;
    return $s;
}

    s/\@(.*)\@/ subst($1) /ge;