Witness the following:

$ perl -Mstrict -Mdiagnostics -E 'my $a = "3"; say $a->[0];'
Can't use string ("3") as an ARRAY ref while "strict refs" in use at -
+e line 1 (#1)
    (F) You've told Perl to dereference a string, something which
    use strict blocks to prevent it happening accidentally.  See
    "Symbolic references" in perlref.  This can be triggered by an @ o
+r $
    in a double-quoted string immediately before interpolating a varia
+ble,
    for example in "user @$twitter_id", which says to treat the conten
+ts
    of $twitter_id as an array reference; use a \ to have a literal @
    symbol followed by the contents of $twitter_id: "user \@$twitter_i
+d".
    
Uncaught exception from user code:
    Can't use string ("3") as an ARRAY ref while "strict refs" in use 
+at -e line 1.
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If you are getting this message, then your code is doing what my code demonstrates. It may take a more complex path to get there, but the result it the same; some variable that you think has an array reference in it actually has the string "3", and you are attempting to dereference it.