An array in double quotes (which equals qq//) is interpolated and the result is ...

print "left @array right";
# try perl -MO=Deparse for that:
print 'left '.join($",@array).' right'
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So you see, perl uses $" as the seperator of the array elements, $" is by default just a single space character, you can simply change that. Use local() to avoid problems with other code, because CGI.pm afaik uses $", too.

local $" = '';
#...
-default => "@source",
#...

#or even

-default => do { local $" = ''; "@source" },

#or better

-default => join('',@source)
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HTH, see perlvar