$ perl
if ( '_$x_' =~ '$x') { print "match 1" }                      
if ( '_$x_' =~ /\$x/){ print "match 2" }
__DATA__
match 2

##</code><code>##

perl -MO=Deparse 
if ( '_$x_' =~ '$x') { print "match 1" }                      
if ( '_$x_' =~ /\$x/){ print "match 2" }
__DATA__
if ('_$x_' =~ /\$x/) {
    print 'match 1';
}
if ('_$x_' =~ /\$x/) {
    print 'match 2';
}
__DATA__
- syntax OK

##</code><code>##

perl -MO=Concise
if ( '_$x_' =~ '$x') { print "match 1" }                      
if ( '_$x_' =~ /\$x/){ print "match 2" }
__DATA__
g  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 3 -:1) v:{ ->3
-     <1> null vKP/1 ->9
5        <|> and(other->6) vK/1 ->9
4           </> match(/"$x"/) sKS/RTIME ->5              <-- INTERPOLATED
3              <$> const[PV "_$x_"] s ->4
-           <@> scope vK ->-
-              <0> ex-nextstate v ->6
8              <@> print vK ->9
6                 <0> pushmark s ->7
7                 <$> const[PV "match 1"] s ->8
9     <;> nextstate(main 5 -:2) v:{ ->a
-     <1> null vKP/1 ->g
c        <|> and(other->d) vK/1 ->g
b           </> match(/"\\$x"/) sKS/RTIME ->c            <-- NO INTERPOLATION
a              <$> const[PV "_$x_"] s ->b
-           <@> scope vK ->-
-              <0> ex-nextstate v ->d
f              <@> print vK ->g
d                 <0> pushmark s ->e
e                 <$> const[PV "match 2"] s ->f
- syntax OK