You can use the -F option to specify the split pattern for each line. If the pattern is found at the beginning of the string, $F[0] will be the empty string.

Edit: so replacing the empty element with 'ID' can be done like this: perl -lF"\t" -E 'print join qq<\t>, map { $_ || q<ID> } @F', which works because there are no 0 in your values.

Is there a way to print something into this first element?

I suspect you're after something more complex, but on the face of it, this seems to answer that question:

C:\test>type junk.dat
        A       B       C       D       E
A       1       1       1       1       1
B       1       1       1       1       1
C       1       1       1       1       1
D       1       1       1       1       1
E       1       1       1       1       1

C:\test>perl -plae" $.== 1 and $F[0] eq 'A' and $_ = 'ID' . $_" junk.d
+at
ID      A       B       C       D       E
A       1       1       1       1       1
B       1       1       1       1       1
C       1       1       1       1       1
D       1       1       1       1       1
E       1       1       1       1       1
[download]

Thanks! This works well (after replacing " with '). It is slightly more complex in that $F[0] will not always eq 'A', so this could also be used:

perl -plae '$.== 1 and $_ = 'ID' . $_'

Is there a way for it to simply check for any string (non-delimiter character) at all?

Eily called it right:

C:\test>type junk.dat
        A       B       C       D       E
A       1       1       1       1       1
B       1       1       1       1       1
C       1       1       1       1       1
D       1       1       1       1       1
E       1       1       1       1       1

C:\test>perl -F\t -plae"$.== 1 and $F[0] eq '' and $_ = 'ID' . $_" jun
+k.dat
ID      A       B       C       D       E
A       1       1       1       1       1
B       1       1       1       1       1
C       1       1       1       1       1
D       1       1       1       1       1
E       1       1       1       1       1
[download]

---

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If you literally only need to create first entry, don't split?

perl -pe 'BEGIN { print "ID"; }' /tmp/test.dat
[download]

This is an excellent example of finding the X in an X/Y problem. You've done a good job of separating the problem from the proposed solution on even a fairly simple example program. I'm going to bookmark this to reference for future discussions of this sort.