=== 1 ===
perl -MO=Deparse -e "$_='abc@de';print if(/[_@]/)"
$_ = 'abc@de';
print $_ if /[_\@]/;
-e syntax OK

perl  -e "$_='abc@de';print if(/[_@]/)"
abc@de

=== 2 ===
perl -MO=Deparse -e "$_='abc@de';print if(/[@_]/)"
$_ = 'abc@de';
print $_ if /[@_]/;
-e syntax OK

perl -e "$_='abc@de';print if(/[@_]/)"
/[]/: unmatched [] in regexp at -e line 1.

=== 3 ===
perl -MO=Deparse -e "$_='abc@de';print if(/[_@a]/)"
In string, @a now must be written as \@a at -e line 1, near "[_@a"
-e had compilation errors.
$_ = 'abc@de';
print $_ if /[_@a]/;

=== 4 ===
perl -e "@a[0]='z';@a[23]='oob';$_='abc@de';print if(/[_@a]/)"
abc@de