In general

• any Perl function or subroutine that returns an array can return a count instead by prefixing it with scalar. Thus:
  • scalar @aFiles returns the number of items in @aFiles
  • scalar grep returns the number of matches instead of the actual matches.
  • scalar keys returns the number of keys in a hash rather than the actual keys.
• if you need to match the entire value, start your regex with ^ and end it with a $. /a/ matches any string with the letter a. /^a$/ matches only the string "a".
• next if ... lets you skip a loop iteration if whatever comes after "if" is true

So applying this to you little code snippet:

for my $k (sort keys %$hFiles) {
  # ^$k$ forces $k to match the entire value of each entry in @aFiles
  # -  ^ forces match at start of entry
  # -  $ forces match at end of entry
  # -  ^$k$ expands to whatever is in $k, 
  #    i.e. $k="x", then expands to ^x$
  #
  # next if scalar grep... will go back to start of loop whenever one
  # or more values match
  # - scalar grep - returns number of matches
  # - next goes back to start of loop

  next if scalar grep( /^$k$/, @aFiles);
  print "key=$k\n";
}
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For more information, see scalar and perlre

next if scalar grep( /^$k$/, @aFiles);

if already imposes scalar context on its operand.

if (my @matching_files = grep /\Q$key/i, @files) {
   ...
}
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I want to enter the loop when the grep *fails*.The below condition enters the loop when the grep passes

if (my @matching_files = grep /\Q$key/i, @syncsource_plf) {
...
}
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if ( not grep { condition } list ) {