Too many curlies. toolic's advice is good, but you can safely remove even more:

s/$thing/$translate{$thing}/g;
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Be careful, though, if one translation results in a word that another translation might translate again.
Sometimes, avoiding the inner loop by constructing the regex leads to a faster solution:

my $re = join '|', keys %translate;
$re = qr/($re)/;

while (<>) {
    s/$re/$translate{$1}/g;
    print;
}
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You might need to add some \b boundaries to the regex so it does not match in the middle of a word.