What does ${wfid} mean in here ? Is it same as $wfid ? If yes then why use this syntax ?

$filelist = "$iwhome/tmp/filelist-${wfid}txt";
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Without the curlies, you'd have a different variable: $wfidtxt:

$filelist = "$iwhome/tmp/filelist-$wfidtxt";
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Refer to Scalar value constructors

Is file handler FILELIST adding members to $filelist ?

Generally, you need not use &. Refer to Prototypes for gory details.

While my esteemed predecessors have done a good job of answering your direct questions, perhaps I can shed some light on a few stylistic questions you didn't ask, but may still find informative.

The line:

is problematic for a couple of reasons:

• The open might fail (for example, if the file does not exist), so you need to check the return.
• The two-argument open (with the append (>) symbol in the same string as the filename) is a potential security risk; see the old Two-arg open() considered dangerous discussion for more info.
• The filehandle, FILELIST, is in the global symbol table. It's better form to create a lexical variable instead with my.

So, instead, I would write:

(UPDATED thanks to AnomylousMonk's good catch of a very carelessly chosen variable name)

    open my $fh, '>', $filelist
        or die "Can't open $filelist: $!";
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If it fails, the program will exit with an informative error code. You could instead wrap the open call in an if statement if you want to do something else.

Purely personal preference on this next one, but your entire foreach { ... } loop could be more concisely written:

open my $filelist, '>', $filelist 
        or die "Can't open $filelist: $!";
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This is simply an inadvertance, but using a new lexical  my $filelist for the filehandle in the open statement masks the file name in the old lexical of the same name (and also generates a warning if warnings are enabled, which is strongly recommended). I would write something like:

>perl -wMstrict -le
"my $filelist = 'xyzzy';
 open my $filehandle, '>', $filelist or die qq{opening '$filelist': $!
+};
 print $filehandle $_ for qw(foo bar baz);
 close $filehandle or die qq{closing '$filelist': $!};
"
>type xyzzy
foo
bar
baz
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${wfid}

Sometimes it's useful to be clarify what's part of the variable name and what isn't, especially in strings. In this case, it wasn't needed but if it was ${wfid}_1.txt, it would be necessary.

file handler FILELIST adding members to $filelist

Not quite. FILELIST represents the file (given in $filelist) and the "foreach" loop is adding to the file identified by FILELIST. Maybe it's just semantics but that's the way I read it.

why is it preceded by &