echo stuff | perl -ne 'BEGIN{ $x=shift @ARGV; } print "x=$x\n"' myarg
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Is this like what you had in mind?

I tried this but somehow $x seems to become undef after the first time through the implicit loop.

$ cat gibberish
This is a couple
of lines of crud
$ cat gibberish | perl -Mstrict -Mwarnings -ne '
> my $x;
> BEGIN {$x = shift @ARGV; print qq{BEGIN - $x\n}}
> print qq{x = $x\n$_};' myarg
BEGIN - myarg
x = myarg
This is a couple
Use of uninitialized value in concatenation (.) or string at -e line 4
+, <> line 2.
x = 
of lines of crud
$
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I wonder what is causing this.

Cheers,

JohnGG

Don't declare the $x — it's just a one-liner. Perl wraps a while(<>) { ... } loop around the code you put there (really!), so it's a new $x for every line in the file.

See for yourself:

perl -n -MO=Deparse -e 'my $x' foo
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produces:

LINE: while (defined($_ = <ARGV>)) {
    my $x;
}
-e syntax OK
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Thanks a lot. Indeed, with the BEGIN block, it now works.

In case someone is curious, I needed it in a shell script for a convoluted situation: to be able to un-mount a disk cleanly, I needed a smart sort on the mounted partitions of the disk.

# $dest was set to "sdb" through a shell argument 
mounted_disks=$(mount | perl -nae 'BEGIN{$d=shift}; push @m, $F[2] if 
+m{/dev/$d}; END{print join(" ", sort {length($b)<=>length($a)} @m)}' 
+$dest)
umount -l $mounted_disks
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Thanks to Perl and the Perl Monks, it works.

You don't really need a loop for that, you could do it like this:

mounted_disks=$(mount | perl -le'$d = shift; print join " ", sort { le
+ngth( $b ) <=> length( $a ) } map m{/dev/$d} ? (split)[2] : (), <>' $
+dest)
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Or even move the mount command into the perl code:

mounted_disks=$(perl -le'$d = shift; print join " ", sort { length( $b
+ ) <=> length( $a ) } map m{/dev/$d} ? (split)[2] : (), qx(mount)' $d
+est)
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Maybe something like this?

echo stuff | perl -ne '$x=shift; print "x=$x\n"' - myarg
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It's always a good idea to tell what the expected outcome is :)

With one line of input on stdin (like your echo), this produces the same result as bart's suggestion. With more lines on stdin, however, you'd get different behaviour. Assuming that stuff is a file containing the two lines "foo" and "bar", then when doing

cat stuff | perl -ne '$x=shift; print "x=$x\n"; print "<>=$_"' - myarg
+ myarg2 stuff
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you'd get

x=myarg
<>=foo
x=myarg2
<>=bar
x=
<>=foo
x=
<>=bar
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i.e. you'd shift as many arguments from @ARGV as there are lines on stdin. For the remaining arguments, Perl would try to open the respective filename.