You can accomplish this fairly easily using negative Look Around Assertions.

#!/usr/bin/perl
use strict;
use warnings;

my $test_string = "foo l?est sé?jou ? foo ";
$test_string =~ s/(?<!\s)\?(?!\s)/'/sig;
print "$test_string\n";
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The expression will fail when the character before ((?<!...)) the question mark is white space and when the character after ((?!...)) the question mark is white space. This will substitute if there is a leading or trailing question mark. These are zero-width assertions, so you don't have to worry about capturing. See perlre for details.