Looks like you're trying to dereference gmtime() in package main::. You're in main:: by default, and you're not specifying a package for it. The builtin Perl functions live in package CORE::.

If you really need to specify the whole address of a core function, try &CORE::gmtime. (I usually don't use the &subroutine-name syntax, as I think subroutine-name() looks better.)