I don't know how 1 while s/// is clearer than s///g.

What's worse, it does not work correctly:

my %map = (
    F => "|",
    S => "^",
    T => "&",
    R => "~",
    E => "\\"
);

$_ = "\\E\\S\\";
s/\\([FSTRE])\\/$map{$1}/g;
print "s///g: [$_]\n";

$_ = "\\E\\S\\";
1 while s/\\([FSTRE])\\/$map{$1}/;
print "1 while s///: [$_]\n";

__END__
s///g: [\S\]
1 while s///: [^]
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\E\S\ gets translated to \S\, as per the OP's spec, but the 1 while s/// solution then blithely goes on to translate that as well. Oops.

(Note that there's a bunch of minor mistakes in your code. The last slash in your s/// is missing and (FSTRE) should be ([FSTRE]).)