map 'returns' a result for each element processed. The result is the last value of the last statement in the map block and may be a list. Consider:

#!/usr/bin/perl
use strict;
use warnings;

my @lines = qw(<tag>A</tag> <TAG>B</TAG> <tag>C</tag>);

for my $expr (
    'lc',
    'lc($_), 1',
    '$_=lc($_); s/<tag>//; s/<\/tag>//; $_ => 2'
    ) {
    my @values = eval "map {$expr} \@lines";

    print join("\n   ", "map {$expr} =>", @values), "\n";
}
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Prints:

map {lc} =>
   <tag>a</tag>
   <tag>b</tag>
   <tag>c</tag>
map {lc($_), 1} =>
   <tag>a</tag>
   1
   <tag>b</tag>
   1
   <tag>c</tag>
   1
map {$_=lc($_); s/<tag>//; s/<\/tag>//; $_ => 2} =>
   a
   2
   b
   2
   c
   2
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That result can be a list (as shown by almut