warn takes a string as argument, so just add the coordinates you want.

Just let the string end with a newline "\n" to avoid other numbers to be added automatically.

warn "Warning at FILE.pl line 42, <> line 666.\n";
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et voila!

Warning at FILE.pl line 42, <> line 666.
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besides:

The example code you gave is nonsense, because foreach slurps the whole file, line-numbers are lost.

Better use while !

Otherwise please post code demonstrating your problem and the expected output.

update

If you want the first FILE and NUMBER to be the correct current ones, put your warn into a sub and use caller to get them, before constructing your string.

Alternatively use Carp to report from the callers context.