The fact that $1 is a pre-declared variable, and that @1 is not does not mean that @1 could not have been a variable, and consequently, that $1[... should not be seen as the beginning of an indexing into the @1 array. It really probably ought to be a strict violation first, since @1 isn't special, but is an identifier, and subject to the rules of parsing that apply to sigil-preceded identifiers when interpolated into a regexp quote-like construct.

perl -E 'our @array = (1,2,3); *1=\@array; say for @1;'
1
2
3
[download]

Here we cheated a little, using the typeglob syntax to create an array by the name of @1. Well, really a symbol for an array named @1, which happens to be an alias to @array in the symbol table. And it is shown to work. With that in mind, there's no reason that m/something$1[.../ should not be seen as the beginning of the indexing into an array element for interpolation within a regexp. Disambiguation can be achieved by wrapping the symbol portion of $1 in curly braces, as in ${1}:

# $1 will be undef, so not relevant to the pattern. [0] will be a sing
+le-character character class, so matches "0".
perl -E 'our @array = (1,2,3); *1=\@array; say for @1; say "yes" if "0
+" =~ m/.?${1}[0]/;'
1
2
3
yes
[download]

Otherwise, this will be variable interpolation:

# $1[0] is seen as interpolation, placing the value of "1" into the re
+gexp, which matches with the "1" in our target string.
perl -E 'our @array = (1,2,3); *1=\@array; say for @1; say "yes" if "1
+" =~ m/.?$1[0]/;'
1
2
3
yes
[download]

The unfortunate part here is that strict appears to be special cased for the numeric-variable symbols rather than specifically for scalars.